Leiden's Galaxies Journal Club

A Compact Early-type Galaxy at z = 0.6 Under a Magnifying Lens

2010-11-12T12:10:00.003+01:00

By Auger et al.

Figure 1. Keck LGS-AO imaging of the EEL SDSSJ1347-0101 (left). The image is well modelled using a singular isothermal ellipsoid mass distribution for the lensing mass, a single Sersic component for the lensing galaxy, and a two-component de Vaucouleurs and Sersic profile for the background source (right), while a single-component fit to the source leaves a ring of residual flux (centre); the scaling of the two residual plots is set to saturate at 3 times the background noise level. The Kp-band imaging reveals an observed source magnitude of 17.6 but this is corrected to 20.3 when the magnification from lensing is taken into account; the inferred stellar mass of the source is 10^10.9M⊙ and the half-light radius of the two-component surface brightness model is 0.′′16 or 1.1 kpc at the redshift of the source.

Rapid QSO shutdown in Hanny's Voorwerp

2010-11-04T12:02:00.002+01:00

Schawinski et al (2010, http://arxiv.org/abs/1011.0427) presents interesting X-ray observations of Hanny's Voorwerp. They argue that the light from the Voorwerp (the green fuzz above) is caused by ionizing radiation from an AGN in the center of the host galaxy IC 2497. However their X-ray data show only a weak AGN, much too weak to explain the emission lines in the Voorwerp they argue (by 2-4 orders of magnitude). The inference is thus that the Voorwerp is a light echo of a QSO phase that terminated < 70,000 years ago. This is a rapid turn-off and more extreme than what you would expect by scaling up an X-ray binary for instance.

Title: Gas accretion as the origin of chemical abundance gradients in distant galaxies

2010-10-15T12:01:00.004+02:00

In Cresci et al. (arXiv: 1010.2534) they find 'inverse' metallicity gradients for three rotationally supported star-forming galaxies at z=3. They conclude that the central gas has been diluted by the accretion of promordial gas, as predicted by 'cold flow' models. In Jones et al. (arXiv: 1010.1538) they find a 'normal' metallicity gradient for a lensed z=2 star-forming galaxy. While the physical gradient is considerably steeper than that observed in local galaxies, in terms of the effective radius at that epoch, the gradient is similar. They conclude that subsequent growth occurs in an inside-out manner with the inner metallicity gradient diminished over time due to radial mixing and enrichment from star formation.

MEASUREMENT OF A METALLICITY GRADIENT IN A Z = 2 GALAXY:

2010-10-15T11:55:00.010+02:00

Jones et al. present near-infrared imaging spectroscopy of the strongly-lensed z = 2.00 galaxy SDSSJ120601.69+514227.8 (‘the Clone arc’).

FIG. 1: (Left) Hubble Space Telescope color composite image of the Clone arc. A1-5 represent multiple images of the z=2.00 source; the critical curve is shown in red. The OSIRIS pointings are indicated by the two green rectangles offset by 1.8 arcseconds in the East-West direction. Foreground lensing galaxies are labeled as G1-4. (Right) Distribution of the key emission line fluxes in multiple images A3-5 in units of 10−18 erg/s/cm2 (see text for discussion of optimum samplig). North is up and East is to the left. The critical line passes through the A3 and A4 components such that a small part of the arc (including A5) is imaged 4 times in the OSIRIS field. The remainder of A3/A4 is imaged twice in the OSIRIS field.

From these and other results they conclude:

We find a strong radial gradient in both the [Nii]/H and [Oiii]/H ratios indicating a metallicity gradient of −0.27±0.05 dex kpc−1 with central metallicity close to solar. This suggests that subsequent growth occurs in an inside-out manner with the inner metallicity gradient diminished over time due to radial mixing and enrichment from star formation.

How does galaxy environment matter?

2010-09-17T10:30:00.005+02:00

For some obscure reason blogger does not let me put on my text here fully, so see the first comment for that....

2010-09-10T12:13:00.003+02:00

In 1009.1619 Nilsson et al ('Stellar populations of z=1 LBGs from ACS slitless grism spectra') the authors perform sliless spectroscopy on a small sample of z~1 LBGs and perform SED fitting on these. They compare the SED fitting results to broadband photometry SED fitting and find that the addition of slitless (high S/N, low spectral resolution) spectroscopy helps greatly in narrowing the allowed range of solutions of the stellar population.

The stellar populations are very well fit by a single SSP or by a continuous SFR, a second SSP has a best fit mass of ~0. Stellar masses, ages and extinction are very well constrained (~10^10 Msun, few-few tens of Myrs, 0.5-2 mag A_V), but the metallicity is not constrained at all.

No bulge in the Milky Way?

2010-05-07T12:09:00.002+02:00

In 1005.0385 (Our Milky Way as a pure-disk galaxy - A challenge for galaxy formation), Shen et al. model Milky Way like galaxies with N-body only code. The set up disk galaxies with bars of various strengths and evolve them dynamically for a bit. Then they add realistic bulges of various masses and see what is still feasible, giving the kinetic data of the BRAVA kinetic bulge observations. What they find is that the puffing up of the initially quite thin bar mimicks the presence of a bulge already, if viewed from the location of the Sun. When they include bulges of various masses, they find that the bulge is at most as massive as ~.1 times the mass of the disk. This is, they say, much less then required by 'the classical galaxy formation scenario'.

An x-ray detected cluster at z=1.62

2010-04-23T13:26:00.000+02:00

Tanaka et al. have independently identified the cluster at z=1.62 that I blogged about recently, and have confirmed the redshift using NIR spectroscopy. They have also detected it in the x-ray, thereby upgrading its status from "protocluster" to "cluster" (according to the common usage of those terms), and making it the most distant cluster known.

Dust in DLAs

2010-04-23T12:26:00.004+02:00

Frank & Peroux (http://uk.arxiv.org/abs/1004.3298) has done a study of dust attenuation in damped-Lya absorbers from the SDSS DR7. Using 676 absorbers and a comparison sample of QSOs they manage to put stringent constraints on the amount of dust attenuation in the Lya absorbers and find that on average they have <0.01 in E(B-V). What is nice is that they show that if they use the same selection techniques as others have used in the past, they do recover their detections - so their conclusion is that the average DLA has very little dust but that some specially selected subsamples do show some.

2010-04-16T11:48:00.003+02:00

Peng et al (2010, http://de.arxiv.org/abs/1003.4747) take a data-driven approach to study how star-formation is turned off in different environments/different mass scales using SDSS and zCOSMOS. They combine a number of results but one of the fundaments for their work is the plot above which shows the relative quenching efficiency as a function of local density in several mass bins (top) and as a function of mass in several local density bins (bottom) - this plot is for the SDSS.

They define quenching here to be the number of objects on the red sequence that would have been on the blue sequence in the lowest density environment - so it is all done relative to the lowest density environment.

Their conclusion is that the effect of environment and mass seem to be decoupled and can be treated separately - ie. that there is one source of quenching that depends on stellar mass and one that depends on local density but that they do not interact significantly. They also find the same for zCosmos at z~0.8 and postulate that it extends somewhat higher in redshift.

They are able to fit a range of observables with a very simple model and quantify the relative importance of what they call 'mass quenching' and 'environmental quenching' for galaxies of different mass at different redshifts.

A z=1.82 Analog of Local Ultra-massive Elliptical Galaxies

2010-04-15T11:22:00.006+02:00

This figure by Onodera et al. shows a velocity dispersion (red-filled circle) of a massive galaxy at z=1.82 in comparison to others (i.e. SDDS, Cappellari et al. 2009, van Dokkum et al. 2009). The velocity dispersion was measured from a 4.7 hours spectrum obtained with MOIRCS on Subaru. They find that their observations (morphology, size, and velocity dispersion) are fully consistent with those expected for passively evolving progenitors of today's giant ellipticals. I particularly like their conclusion: "It is clear that many more observations of similar galaxies are required to establish which kind of ETG is commonest at high redshift: either the compact/high-velocity dispersion objects like those found by van Dokkum et al. (2009), or the apparently normal, low- velocity dispersion objects presented in this paper."

The physical origins of the morphology-density relation: evidence for gas stripping from the SDSS

2010-04-09T12:02:00.004+02:00

This figure by van der Wel et al. shows how the axis ratio of quiescent galaxies from SDSS depends on the halo mass and stellar mass. The left panel shows that at lower halo masses, quiescent galaxies with stellar mass (5-10)x10^10 Msun tend to be round, but at higher halo masses quiescent galaxies show a wider range of axis ratios. Most of the quiescent galaxies in high-mass halos are satellites, and so this additional satellite population (which is not present at lower halo masses) tends to have higher ellipticities.

The authors go on to present a simple model that shows how this extra population of quiescent satellites has an ellipticity distribution that is indistinguishable from ~L* spiral galaxies. If I understood it correctly, this model has no free parameters... which makes the agreement with data pretty impressive. So the interpretation is that the satellite galaxies may have been typical field spirals, which had their star formation shut off through some environmental process that did not affect the structural properties. The natural physical explanation is the gradual stripping of gas in the satellite galaxies and their (sub-) halos.

Which all sounds fine to me. But note that the difference in ellipticities only holds over an intermediate range of stellar mass, as shown in the right panel. My first thought is that perhaps the more massive galaxies were already quiescent and already had round profiles before they were accreted. And that at lower masses, all of the quiescent galaxies are satellites that became quiescent through environmental processes which operate with the same efficiency even in lower mass halos.

2010-03-26T11:51:00.002+01:00

In 1003.4018 Schawinski et al take a sample of morphologically selected Early types from SDSS stripe 82 (deeper imaging), and by eye investigate whether they show signs of major mergers. The fraction of mergers is plotted against three measures of time: u-r colour, Emiisionline classification and post-starburst age.

They want to conclude from this seqence that mergers drive AGN, with a delay time of ~500 Myr and say this is one of very few observational indications of the theoretical understanding of AGN feeding by mergers.

The FIR SEDs of z~2 galaxies: evidence for scaled up cool galaxies

2010-03-26T10:48:00.005+01:00

Last week we talked about a paper that calibrated the relationship between the observed 24um (= 8um in the restframe) flux and SFR for z~2 galaxies. This paper by Muzzin et al. looks at a similar issue, but takes a slightly different approach.

The Chary & Elbaz templates describe the infrared SEDs of local galaxies. These templates have a luminosity dependence, such that IR-luminous galaxies have templates that peak at shorter wavelengths, due to increased dust temperatures.

So one method of estimating the SFR of a galaxy is to see which template corresponds to the observed restframe 8um luminosity, assume that the template accurately describes the SED throughout the rest of the IR regime, and to convert the total IR luminosity of the template into an SFR.

The red dashed curves in the SEDs shown above illustrate the procedure. But Muzzin et al. have compared the templates to the observed fluxes at different IR wavelengths for these two galaxies, and obviously the templates don't work very well. But the solid curves, which are templates for low-luminosity galaxies that have been scaled up to match the data points, do provide good fits. This shows that the correlation between IR luminosity and dust temperature that we see in the local universe doesn't work at higher redshifts, and that even IR-luminous galaxies at z~2 can have relatively cool dust. Previous studies that relied on this correlation may have overestimated the SFR by a factor of several.

Muzzin et al. go on to show general agreement between the SFRs as derived from the IR luminosity and from the dust-corrected Halpha luminosity (where the extinction in Halpha is taken to be roughly twice the extinction to the stellar continuum, which in turn is estimated using stellar population synthesis modeling).

The mid-IR luminosities of normal galaxies

2010-03-12T12:07:00.002+01:00

In the last few years there has been a lot of discussion about the effect of TP-AGB stars on stellar mass estimates for high redshift galaxies. Such stars produce a large fraction of the rest-frame NIR light from youngish (~1 Gyr old) stellar populations, so if observations at these wavelengths are used when fitting stellar population models, it is important to take these stars into account. However, given all of the modeling and observational uncertainties associated with our understanding of TP-AGB stars, we are a long way away from being able to robustly incorporate them into stellar population synthesis (SPS) models.

This letter by Dan Keslon and Brad Holden looks at another important effect of TP-AGB stars: their contribution to the MIR luminosity, which is frequently used as an indicator of the star formation rate. The authors estimate contribution to the MIR luminosity by adding the observed K-MIR colors for Galactic TP-AGB M and C stars to the expected contribution to the K-band luminosity that comes from Maraston's SPS model. This figure shows some of the results. The upper blue lines show the maximum contribution to the MIR, which occurs when the TP-AGB C stars dominate (although this comparison must depend somewhat on the star formation history assumed in the models). It thus appears that the MIR luminosity of galaxies is consistent with being entirely due to the TP-AGB stars.

There's a lot of stuff in this (very long) letter, but what is most striking is their figure 2d, which I haven't shown here. That figure shows that the authors are able to reproduce the observed correlation between 24um luminosity and star formation rate from Chary & Elbaz (2001) very well, but only when the SFR is averaged over the last 1.5 Gyr (which is when the TP-AGB stars are important). But I do find this result a little weird, since presumably that correlation was made against SFR indicators that are sensitive only to more recent SF. Also, it would suggest that very little of the MIR luminosity comes from dust that is not immediately surrounding TP-AGB stars.

The Merger-Driven Evolution of Massive Galaxies

2010-03-05T12:16:00.001+01:00

Robaina et al. estimate the contribution of major mergers to the growth of the red sequence at M>10^11 Msun from z~1 to z~0. To do this, they estimate the merger rate of all galaxies (i.e. both red and blue) more massive then 5x10^10 Msun using the fraction of galaxies in close pairs in COSMOS and COMBO-17, and make the assumption that the remnants of all such mergers lie on the red sequence (or that they move onto the RS very quickly). The data points in this figure show the observed evolution in the number density of M>10^11 Msun RS galaxies, and the curve shows the predicted evolution due to their major merger estimates.

The agreement is impressively good. But given the uncertainties (in the number density evolution, which is uncertain in part because of the uncertainty in the M/L evolution, as well as the uncertainties involved in getting a merger rate from a correlation function), it doesn't seem like you can draw very strong conclusions. Additionally, as the authors mention, they somewhat underestimate the growth due to mergers because they would not count a merger between e.g. an 6x10^10 Msun galaxy and an 4x10^10 Msun galaxy.

The authors also find that a present day M>10^11 Msun galaxy has undergone 0.5 major mergers between M>5x10^10 Msun galaxies since z=0.6, and 0.7 such mergers since z=1.2.

Anyway, this is a nice work, and points to the importance in major mergers at the massive end. I am somewhat curious about what the growth due to more minor mergers is, especially given that minor mergers are supposed to be what drives the size evolution of the RS galaxies.

A Spitzer-selected galaxy cluster at z=1.62

2010-02-19T12:11:00.000+01:00

Papovich et al. have found a galaxy (proto-) cluster at z=1.62 by searching for overdensities of objects with red 3.6um-4.5um colors. Both star-forming and quiescent galaxies at this redshift will have red colors in these passbands, so this is different than the red-sequence selection used by other groups.

Nonetheless, in the author's words, "this is the highest redshift, spectroscopically-confirmed clustering with a strong, well-defined red sequence" (but note that none of the RS galaxies has spectroscopic redshifts).

High molecular gas fractions in normal massive star forming galaxies at z~1-2

2010-02-12T12:12:00.001+01:00

Tacconi et al.

The growth of massive galaxies since z=2

2009-12-04T12:19:00.001+01:00

From van Dokkum et al. (http://arxiv.org/abs/0912.0514).

The first figure shows the average radial surface density profiles for massive galaxies in five redshift bins (z=2, 1.6, 1.1, 0.6, 0 from bottom to top). The profiles come from stacking galaxies from the NEWFIRM Medium-Band Survey.

The second figure shows how much of the mass growth for these galaxies over 0<z<2 comes from star formation, and from mergers (the growth in mergers is just inferred to be the observed change in stellar mass minus the change in stellar mass that is estimated from the star formation rates)

Restframe UV extinction laws at z~1

2009-11-27T12:21:00.002+01:00

In this paper (http://arxiv.org/abs/0905.4073v4), Conroy plots the observed B-R colors for DEEP2 galaxies as a function of redshift, and overplots the predicted colors for a constant star formation stellar population model using different attenuation curves. The left panel shows that a Milky-Way like curve without the 2175A "UV bump" seems to give the best result.

Multiple populations in MW globular clusters

2009-11-27T11:42:00.004+01:00

In http://uk.arxiv.org/abs/0911.4798, which also came out in Nature this week, Lee et al presents the results of an study of the Calcium abundance in a sample of 8 globular clusters. Why do we care? Well, traditionally globulars are thought to mostly be single stellar populations where all stars formed in a very short time; as Ca is produced by SN II you would expect no variation in Ca abundance in this scenario. But that is not what Lee et al found, indeed they found that in 7 out of their clusters there was clearl evidence for a broadened, or in some cases, double red giant branch. This argues for a more complex formation history for globular clusters and might argue that many of them are the remnant nucleus of accreted dwarf galaxies (this is open to argument though).

The evolving stellar-to-halo mass ratio

2009-11-20T12:14:00.002+01:00

In this paper (not really new one; it was posted to astro-ph in
March), Moster et al. use (something like) an abundance-matching
technique to match galaxies to halos. The paper focuses mostly on
z=0, but they also show results for higher redshifts, where they use
stellar mass functions from Drory and from Fontana.

This figure shows the average stellar mass as a function of halo mass
at different redshifts. I've drawn a line that shows the what a
constant ratio would look like. The highest ratio (which means the
highest efficiency for putting baryons in stars) for the z=0 curve
appears at a stellar mass of log(M)~10.5, and increases with
redshift. Another thing to notice is that the curves evolve strongly
at lower masses, and cross at higher masses. This means that, at
lower masses, galaxies grow in mass much faster than their halos. But
at higher masses halos grow faster than galaxies.

2009-11-06T11:18:00.002+01:00

Formation of late-type spiral galaxies: Gas return from stellar populations regulates disk destruction and bulge growth.

In astro-ph/0911.0891, Marie Martig and Frederic Bournaud report on the growth of bulges in disk like galaxies in a cosmological environment. The zoom in on a Milky-Way like halo in cosmological box that had a quiet merger history, to make it prone to disk formation. They include baryonic physics, including star formation, but excluding supernova feedback. In one simulation they add the mass loss of older stellar populations in a relatively simple way. They let the stars loose an amount of mass that is typical for a Salpeter IMF (~45% of the SSP mass is returned in total). This lost gas mass adds to the disk and makes disk survival (and a smaller bulge fraction) a lot easier. The disk becomes more stable to both internal instabilities and to minor mergers.

The Dependence of Star Formation Rates on Stellar Mass and Environment at z~0.8

2009-10-09T12:19:00.001+02:00

This plot shows recent results from Patel et al., who measured the
masses and SFRs of z~0.8 galaxies in a large field which includes a
cluster. The colored data points show the median mass and SFR of
galaxies in three different density bins, where the density is
calculated from the distance to the 7th-nearest neighbor. Galaxies in
higher densities have lower sSFRs, even at fixed mass. The black points
are values from Maaike's general field sample at similar redshifts.

Serendipity in Astronomy

2009-08-21T12:18:00.003+02:00

This article by A. C. Fabian discusses some aspects of the role serendipity plays in astronomical research. The plot above makes the useful point that, even with all the luck in the world, it won't do any good unless you're prepared enough to recognize the good luck and exploit it. As Fabian says, "What is generally needed is for luck to strike someone who is prepared, in the sense that they appreciate that something novel has been seen."

I would add that real instances of pure dumb luck don't happen very often. What happens much more frequently is that somebody was in the right place at the right time, and was attentive enough to notice something interesting. But being in the right place at the right time
frequently takes a lot of work; you have to write the telescope proposal in the first place, or have to have gained access to the right kind of data, talk to the right person, etc., etc. You may do all of these things with a particular aim in mind (to investigate a "known unknown"),
but lucky people probably do these things in the hope of noticing something interesting (an "unknown unknown").

And, of course, being attentive isn't a matter of pure luck either. In other words, I suspect that in most cases people create their own luck. This kind of luck also plays some role in many conventional scientific advances; at least in astronomy, when you begin a project, you frequently can't predict with great accuracy what is going to come out of it, or what the most interesting results will be... so there will always be an element of serendipity.

2009-07-24T11:49:00.002+02:00

D'Onghia, Springel, Hernquist & Keres, "Substructure depletion in the milky way halo by a disk". The put a disk in a cosmologically simulated halo, by hand and observe a lot of stripping of the mass of subhaloes by disk shocking. This reduces the amount of substructure in the inner halo and may help solving the missing satellite problem...

Narrow-line AGN and their host galaxies

2009-07-10T12:14:00.000+02:00

from Greene et al., arXiv:0907.1086

In this paper several aspects of 0.1<z<0.4 narrow-line (obscured) AGN
and their host galaxies are investigated. The sample was selected
from the SDSS and higher-quality follow-up spectra were taken with
Magellan; derived properties include AGN line widths, stellar velocity
dispersions, and Eddington ratios.

The plot above is particularly notable, showing the [OIII] width (i.e.
gas velocity dispersion) compared to the stellar velocity dispersion.
There's no apparent correlation, indicating that gas in the galaxy is
very much out of equilibrium with the stars; from this the authors
conclude that the AGN must be affecting the gas properties on a
galaxy-wide scale.

The diversity of type 1a supernovae from broken symmetries

2009-07-10T11:53:00.001+02:00

Stellar mass growth over cosmic time

2009-07-10T11:05:00.003+02:00

Several authors have found that the SFR of star-forming galaxies is approximately proportional to stellar mass to the first power, and that the constant of proportionality decreases with redshift. Using such a relation, it is straightforward to parameterize the growth of a galaxy
(ignoring major mergers) given it's observed redshift and stellar mass.

This letter by Alvio Renzini, discusses this issue. Renzini takes a recent parameterization of the SFR from the literature, and estimates how a galaxy at z=3 (when the universe was 2Gyr old) will grow. The upper red curve in the plot above shows the growth of stellar mass for the published parameterization... but obviously a typical observed galaxy can't grow in mass by 5 orders of magnitude from z=3 to z=0. So the lower red curves show how the growth will occur if you reduce the SFR at all redshifts by a factor of \eta.

The letter goes on to briefly discuss how environment, mergers, and morphological transformations/quenching will affect galaxy growth.

Velocity dispersion at z~2

2009-06-19T10:47:00.001+02:00

from van Dokkum, Kriek, & Franx 2009, http://arxiv.org/abs/0906.2778

This paper presents the first directly-measured velocity dispersion of a compact quiescent galaxy at z~2. A dispersion of 510 (+165, -95) km/s is measured by fitting galaxy templates to an ultra-deep near-IR (Gemini) spectrum; the dispersion uncertainty is calculated through Monte Carlo simulations.

Such a high dispersion is unparalleled in the local universe, as shown in the figure above. Candidate high-sigma objects were already known, but the dispersions in these cases were indirect (inferred from masses and radii) and so it was conceivable that systematic effects could have been at play. Reassuringly, the stellar masses inferred from SED fitting and the velocity dispersion are in decent agreement with each other, so the previous results apparently are not just due to severely overestimated masses.

A new type of stellar explosion

2009-06-12T11:23:00.002+02:00

From Perets et al. (arXiv:0906.2003) Caption reads:
"Comparison of the SN 2005E ejecta mass and luminosity with other SNe [SNe Ia, squares;
SNe Ib/c, × marks; SNe II, circles]. The lower panel shows the total ejecta mass inferred for SN 2005E, which is the lowest inferred ejecta mass found for any SN, based on nebular spectra. Its position in the luminosity vs. ejecta-mass phase space is unique, suggesting it is not a member of currently well-known SN families. The middle panel shows the Ni mass inferred for SN 2005E. The small Ni mass inferred for SN 2005E is consistent with its low luminosity, although somewhat lower than might be expected from the extension of the observed Ni mass-luminosity relation observed for other SNe (dashed line and formula). The upper panel shows the Ni ejecta mass fraction MNi/Mtotal inferred for SN 2005E. The sources from which the SN data were collected are listed in the SI, Section 8."

This is a supernova that doesn't match any of the known classes. It was not found in a star forming region, so core collapse seems unlikely. It also ejected significantly less mass than any known SNIa. It has a very high Ca yield.

First direct metallicity at z>1

2009-06-08T10:57:00.002+02:00

from Yuan & Kewley, http://arxiv.org/abs/0906.0371

The authors use MOIRCS on Subaru to obtain a near-IR (rest-frame optical) spectrum of a strongly-lensed z=1.7 galaxy, and detect the [OIII] 4363A line, the first such detection at z>1. This line provides a direct measurement of the oxygen abundance; the above plot shows this measurement (red) compared to z>2 galaxies (from Erb et al.; black points) and the local relation (dashed line). While the z>2 data suggested a steeper slope in the metallicity-mass and metallicity-luminosity relations at high redshifts, with the new data point it actually doesn't look much different from at z=0. Of course, caveats about different techniques (the z>2 metallicities were not determined with the same technique) and redshifts apply.

AGN activity in nearby galaxies

2009-06-08T10:32:00.001+02:00

from Goulding & Alexander, http://arxiv.org/abs/0906.0772

The authors describe a volume-limited survey of all (64 in total) IR-
luminous (L_IR>3e9) galaxies within 15 Mpc with Spitzer-IRS. The
goal is to look for the [NeV] line at 14um, which is considered to be
an unambiguous tracer of AGN activity since the ionization potential
of this line is generally too high to produce in HII regions.

The BPT diagram above shows SDSS galaxies (faint grey points),
galaxies from their sample with no detected [NeV] (black squares),
and those with detected [NeV] (red squares). Dividing lines between
star-forming galaxies, LINERs, and Seyferts are also overplotted.
Although only 7 galaxies strictly meet the "Seyfert" classification
based on their optical emission lines, 17 show [NeV]; the authors
conclude that the BPT diagnostic misses more than half of AGN
activity in IR-luminous galaxies. However, most of the "optical non-
AGNs" classify as LINERs, so it seems a bit much to say they were
totally "missed" by the BPT diagnostic. Also, in a subsequent plot
it's apparent that most of these near-IR AGN are extremely weak -
between 1-5% of the total galaxy luminosity. However, there's
marginal evidence for a correlation between AGN activity and L_IR
(hence star formation) in this sample.

2009-06-05T11:21:00.002+02:00

In 0906.0590, Kistler et al investigate "The star formation rate in the reionization era as indicated by gamma-ray bursts". They make a compilation of high redshift long gamma ray bursts, deduce a correction factor as a function of redshift (at intermediate redshifts) between the rate of GRBs and the SFR and calculate the SFR(z), from GRBs. The result is the Lilly-Madua plot shown. The upper yellow points are results from the GRBs. Note that in all four bins there are just a few (1 in the last) GRBs used for the calculation. The grey points are the well known Hopkins & Beacom (2006) compilation, while the other, higher redshift, coloured points are deductions from UV luminosity functions from LBGs (Bouwens et al 2008) and Lyman alpha emitters (Ota et al 2008)

The grey lines with positive slope are the SFRDs necessary to keep the universe ionized according to Madau et al 1999. This seems to indicate that the star formation alone is enough to keep the universe ionized from z~8.

Clustered star formation as a natural explanation of the Halpha cutoff in disc galaxies

2009-05-08T11:28:00.003+02:00

From Pflamm-Altenburg and Kroupa (arXiv:0905.0898v1)
Caption reads:
"The Hα-luminosity surface density versus the total gas surface density observed for seven disc galaxies15 averaged over annuli at different galactocentric radii is plotted (black squares) after correcting for photon leakage from H ii regions (see Supplementary Discussion). These galaxies have a mean star formation rate of SFR=6.9 M⊙ yr−1 (3.2 – 16.4 M⊙ yr−1 )2, 15 , a mean total gas mass of Mgas = 2.1 · 1010 M⊙ (0.6 – 3.6 · 1010 M⊙ )2, 15 and a mean scale length of rd = 4.4 kpc (3.9 – 5.2 kpc)25–28 . These mean values define our model standard disc galaxy. For a choice of γ = 2 the LIGIMF-theory predicts an ΣHα -Σgas relation which matches the observations excellently (solid line). Note that the underlying true star-formation density as derived from UV observations1 is directly proportional to the gas surface density (N = 1) and is shown after converting it into an Hα surface luminosity using the wrong linear Kennicutt Hα-SFR relation2, 29 (dashed line) and shows the expected ΣHα -Σgas relation based on the classical picture which is in disagreement with the observations."
Basically, since stars form in clusters, and because you have lower mass star clusters at lower densities, you expect relatively less massive stars and therefore a Hα cutoff

The statistical nature of the brightest cluster galaxies

2009-04-24T11:41:00.002+02:00

The brightest cluster galaxies (BCGs) lie at the center of clusters, and have very different properties than normal early-type galaxies. They have flatter brightness profiles, with envelopes that extend well out into intra-cluster space, and have extremely large stellar masses. Additionally, the luminosity function of cluster galaxies tends to show a bump at large luminosity, which corresponds to the BCGs. Obviously, BCGs must form differently than other early-types. But one can still ask the question, are BCG luminosities consistent with being drawn from the overall cluster population?

This issue has been visited several times in the past, and now most recently by Lin, Ostriker, & Miller using clusters detected in SDSS. Their basic method is to create mock clusters by scrambling the galaxies among the observed clusters, and then to compare the luminosities of the brightest galaxies in the mock clusters to the luminosities of the actual BCGs. The purple circles in the top panel of this figure show the observed BCG luminosities as a function of total cluster luminosity. The squares show the BCG luminosities using a running mean, and the green crosses show the running mean computed from the mock clusters. It appears that BCGs are more luminous than expected based on the mock clusters, a difference that becomes more apparent at high luminosities.

This conclusion is also apparent in the bottom panel, which shows the difference between the observed BCG luminosities and the the expected luminosity (filled purple symbols), compared to the difference observed in one realization of the mock cluster sample (open red).

The authors conclude that, in a flux limited sample of LRGs, BCGs will become more dominant at high redshifts. So baryonic accoustic oscillation studies will have to take this into account.

A NEW TEST OF THE STATISTICAL NATURE OF THE BRIGHTEST CLUSTER GALAXIES

2009-04-24T11:19:00.003+02:00

In arXiv:0904.3098 Lin, Ostriker & Miller use a cluster catalog, assemblked from SDSS DR5 to examine the question whether or not BCGs are just the statistical extremes from the luminosity distributions of cluster galaxies, or whether they are really distinct objects. They do a very simple test: they throw together all cluster galaxies, and randomly sample luminosities from this distribution, adding up to the total luminosities of clusters. They then compare the statistic d in this plot. d is the difference in log between the mean luminosity of observed(blue)/sampled(red) BCGs and of 200 MC realizationsof this process. The red line therefore should be centered on zero, and it is. The offset of the blue histogram is significant in the total sample (494 clusters) and in the sample of bright clusters (124 clusters). FOr the total sample they calculate a probablity of less than 0.8% that the distributions come from the same parent distribution, for the bright subsample it is 0.03%. For low luminosity clusters there is no significant deviation between the two (P = 54.5%). They therefore claim that (at least for the bright ones) BCGs must evolve distinct from the rest of the galaxy population, making them brighter than their non-BCG cluster companions.

Early Assembly of the Most Massive Galaxies

2009-04-17T12:12:00.002+02:00

In this Nature paper, Collins et al. compare the stellar masses of brightest cluster galaxies (BCGs) at z=1-1.5 to BCGs at z~0 and to the predictions from semi-analytic models. This figure shows the observed masses (red circles; the red cross shows the mean value) compared to the mean mass for a low-redshift sample (dashed line). It also shows the masses of BCGs in the semi-analytic models (grey diamonds are the masses of individual galaxies, and the black circles are the mean). It seems that the high-redshift BCGs have already accumulated essentially all of their mass, with very little of the additional growth that is predicted by the models.

Interestingly, the authors state that two of the BCGs may be undergoing major mergers, which will make them even more massive.

One worry I have is that this analysis relies on matching the high-redshift cluster sample to an appropriate local sample. The authors don't discuss this matching in detail. If the local sample was selected to have the same total cluster mass (which I think is the case), then all that the authors have shown is that the ratio of BCG stellar mass to total cluster mass doesn't evolve with redshift. But, as the authors note (although in a different context), previous studies have already arrived at this conclusion out to z~0.8.

There are other uncertainties in this analysis. The authors use a crude method to estimate the stellar masses (they seem to use only one or two observed bands). Additionally, it is well-known that the total mass of extended low-redshift galaxies is difficult to measure (the authors don't address this issue).

BLAST off

2009-04-09T11:54:00.003+02:00

The non-ironic DVD cover:

The science:

Figure 9 of Pascale et al., http://arxiv.org/abs/0904.1206

The Balloon-borne Large-aperture Submillimeter Telescope (BLAST) released a slew of papers on astro-ph today. This instrument has surveyed 8.7 deg2 in GOODS and ECDFS at 250, 350, and 500 microns. The authors also incorporate IRAC and MIPS data from SIMPLE and FIDEL, and select sources based on 24um flux. The additional wavelength coverage from BLAST allows the authors to better constrain L_IR as well as directly constrain the average dust temperatures of sources as a function of redshift. It turns out that the cosmic IR background can be almost (or entirely) resolved into individual sources, with most of the 70um background coming from z<1 galaxies and the 500um background coming from z>1.

The above plot shows the inferred star formation density evolution from this study (black circles), UV/optical measurements (triangles; dashed error bars have extinction corrections applied). About 70% of their 24um sources had reliable UV/NIR photometric redshifts, and the remaining 30% were estimated through IRAC SED fitting; the grey circles show the SFH with these "IRAC redshifts" excluded. The solid and dashed lines show a luminosity function model with (dashed) and without (solid) taking into account the 24um flux limit of 20 microJansky. As the BLAST measurements fall below the model at thehighest redshifts, the authors conclude that they're missing a population of faint 24um sources.

Black hole M-sigma relation

2009-04-02T15:17:00.003+02:00

This is a guest post from Brent:

Figure 1 from "The M-sigma and M-L Relations in Galactic Bulges and Determinations of their Intrinsic Scatter" by Gueltekin et al. (http://arxiv.org/abs/0903.4897v1):

Caption (Abridged): "The M–sigma relation for galaxies with dynamical measurements. The symbol indicates the method of BH mass measurement: stellar dynamical (pentagrams), gas dynamical (circles), masers (asterisks). Arrows indicate 3sigma_68 upper limits to BH mass. The color of the error ellipse indicates the Hubble type of the host galaxy: elliptical (red), S0 (green), and spiral (blue). The saturation of the colors in the error ellipses or boxes is inversely proportional to the area of the ellipse or box. Squares are galaxies that we do not include in our ﬁt. The line is the best ﬁt relation to the full sample: MBH = 108.12 Msun (sigma/200 km s−1 )4.24. The mass uncertainty for NGC 4258 has been plotted much larger than its actual value so that it will show on this plot."

Interesting Plot showing the latest and most reasonable collection of galaxies with dynamical BH mass estimates, along with a statistically sound investigation of the Msigma relation and its intrinsic scatter. They find a scatter of (log-normal) e_0=0.44±0.06 (0.31±0.06 for ellipticals only), which could be physical or systematic uncertainties in the observations.

High z Low Z Galaxies

2009-03-27T10:05:00.003+01:00

From Salzer et al. (arXiv:0903.3948). Caption reads: Luminosity-metallicity relation for 1300+ low-z KISS galaxies (z < 0.095; small dots) and the 13 [O III]-detected star-forming
KISS galaxies (red squares). The solid line is a linear fit to the low-z galaxies, while the lower dashed line has the same slope but fits the higher-z galaxies with an offset of -1.1 dex.

These galaxies have redshifts in the range 0.29-0.42. Such luminous galaxies at such low metallicities pose problems for galaxy evolution models, as the evolution in metallicity at these redshifts is expected to be mild.

Dissecting the fundamental plane

2009-03-27T09:51:00.004+01:00

In arxiv.org/0903.3603, Graves et al. dissect the red sequence. They use low redshift red sequence galaxies from the spectroscopic sample. They bin in central stellar velocity dispersion, effective radius and residual central surface brightness. Here they show three panels, the fundamental plane (FP) midplane, and stuff with lower central SB, and higher SB. Color indicates the mean luminosity weighted age of the stellar population. The diagonal dashed lines indicate constant dynamical mass. The near vertical structure in the color scale indicates that the mean age is a much stronger function of veloicty dispersion than of effective radius and/or dynamical mass. This implies that the velocity dispersion is a better indicator of stellar age, and thus that stelar velocity dispersion and dynamical mass should not be considered 'the same'.

This result is consistent with merger siulations in which the effective radius depends mainly on orbital parameters of the merging galaxies.

Galaxies at high surface brightness are younger, implying that the thickness of the FP is real (i.e. has a physical origin).

An SMG with A_V>~5

2009-03-19T23:51:00.000+01:00

One of the brightest sub-mm galaxies (SMGs) in the GOODS-N field is completely undetected in deep optical/NIR imaging. Guessing that this object might be part of a known protocluster at z=4.05, Daddi et al. have used the IRAM Plateau de Bure Interferometer to search for CO emission in the expected wavelength range. This figure shows that they found it.

This is quite a remarkable object: the sub-mm emission corresponds to a star formation rate of order 1000 Msun/yr, and the SED suggests stellar mass of roughly 10^11 Msun and A_V in the range 5-7.5. That's a lot of extinction.

Inside-out galaxy growth?

2009-03-13T10:49:00.000+01:00

Figure 1 from Bezanson et al., http://arxiv.org/abs/0903.2044

The authors investigate possible mechanisms for the size and surface
density evolution from z=2.3 to the present. The above figure shows
average stellar density profiles of ellipticals at z=0 (colored
lines, representing different masses) and at high redshift (grey
shaded region). At small radii the profiles match pretty well, but
farther out they diverge rather drastically. This suggests that the
compact high-redshift quiescent galaxies may be the cores of massive
ellipticals in the local universe, growing from the inside out via
minor mergers. (later in the paper the authors show that major
mergers can't produce the observed evolution in the mass-size relation).

Evolution of Lyman alpha halos around HzRG

2009-03-06T12:17:00.004+01:00

Zirm, Dey, Dickinson, Norman
ApJ 2009, 694, L31

Summary: The authors take STIS spectra of 5 z=1 HzRGs. 4 show extended Lyman alpha emission. The Lyman alpha halos that surround these z=1 HzRGs are smaller, and less luminous than those that surround z>2 HzRGs. The authors claim this is due to evolution of these massive galaxies. The z=1 halos are not ionized by shock heating of the infalling gas (as in the high-z halos), but rather by the AGN and star formation.

Weak velocity dispersion evolution

2009-03-06T11:45:00.001+01:00

from Cenarro & Trujillo, http://arxiv.org/abs/0902.4893

We know massive quiescent galaxies evolve strongly in size and
surface density from z=2 to the present. But what about in velocity
dispersion? Here the authors use a stacked spectrum of 13 early-type
galaxies at z~1.6 (total exposure time of 480 hours) from the GMASS
survey. By fitting templates to this stacked spectrum, they derive a
velocity dispersion of 240 km/s at z=1.6, in contrast to the local
value of about 180 km/s. This (shown in the bottom panel) is much
weaker than the size evolution over the same interval (top panel),
but agrees well with models of the sigma evolution via merging
(Hopkins et al 2008; shaded region) and disagrees with the AGN
feedback scenario of Fan et al. (2008; solid line). The authors
attribute the weak evolution to the changing role of dark matter in
galaxies' potential wells: at high redshift the central potential is
dominated by baryons, and at low redshifts it's dominated by dark
matter.

Upper Limit on Dimming of Cosmological Sources by Intergalactic Grey Dust from the Soft X-ray Background

2009-03-06T11:15:00.001+01:00

From Dijkstra and Loeb (http://arxiv.org/abs/0902.4703).

Abstract reads:
"Active Galactic Nuclei (AGN) produce a dominant fraction (~80%) of the Soft X-ray background (SXB) at photon energies 0.5 < E < 2 keV. If dust pervaded throughout the intergalactic medium, its scattering opacity would have produced diffuse X-ray halos around AGN. Taking account of known galaxies and galaxy clusters, only a fraction F_halo <10% of the SXB can be in the form of diffuse X-ray halos around AGN. We therefore limit the intergalactic opacity to optical/infrared photons from large dust grains (with radii in the range a=0.2-2.0 mum) to a level tau_GD<0.15(F_halo/10%) to a redshift z~1. Our results are only weakly dependent on the grain size distribution or the redshift evolution of the intergalactic dust. Stacking X-ray images of AGN can be used to improve our constraints and diminish the importance of dust as a source of systematic uncertainty for future supernova surveys which aim to improve the precision on measuring the redshift evolution of the dark energy equation-of-state. "

The different lines are different models for the dust distribution.

2009-02-20T12:06:00.005+01:00

A role for self-gravity at multiple length scales in the process of star formation

Alyssa A. Goodman1,2, Erik W. Rosolowsky2,3, Michelle A. Borkin1,5, Jonathan B. Foster2, Michael Halle1,4, Jens Kauffmann1,2 & Jaime E. Pineda2

Nature 457, 63-66 (1 January 2009)

Fig: Observation and simulation of molecular cloud L1448. Most of the emission in the L1448 region is contained with large-scale self-gravitating structures, but only a low fraction of small-scale objects show signs of self-gravitation. In the L1448 observations, gravity is significant on all scales, but not in all regions. In contrast, the simulated map implies that nearly all scales, and all regions, should be influenced by gravity (which was ignored in the simulation).

Abstract:
Self-gravity plays a decisive role in the final stages of star formation, where dense cores (size 0.1 parsecs) inside molecular clouds collapse to form star-plus-disk systems1. But self-gravity's role at earlier times (and on larger length scales, such as 1 parsec) is unclear; some molecular cloud simulations that do not include self-gravity suggest that 'turbulent fragmentation' alone is sufficient to create a mass distribution of dense cores that resembles, and sets, the stellar initial mass function2. Here we report a 'dendrogram' (hierarchical tree-diagram) analysis that reveals that self-gravity plays a significant role over the full range of possible scales traced by 13CO observations in the L1448 molecular cloud, but not everywhere in the observed region. In particular, more than 90 per cent of the compact 'pre-stellar cores' traced by peaks of dust emission3 are projected on the sky within one of the dendrogram's self-gravitating 'leaves'. As these peaks mark the locations of already-forming stars, or of those probably about to form, a self-gravitating cocoon seems a critical condition for their existence. Turbulent fragmentation simulations without self-gravity—even of unmagnetized isothermal material—can yield mass and velocity power spectra very similar to what is observed in clouds like L1448. But a dendrogram of such a simulation4 shows that nearly all the gas in it (much more than in the observations) appears to be self-gravitating. A potentially significant role for gravity in 'non-self-gravitating' simulations suggests inconsistency in simulation assumptions and output, and that it is necessary to include self-gravity in any realistic simulation of the star-formation process on subparsec scales.

Explaining Lya Blobs as Cold Streams of Gas in the Halos of Galaxies

2009-02-20T11:58:00.001+01:00

There have been several proposed mechanisms to power the extended diffuse Lyman-alpha emission (Lyman-alpha blobs, or LABs) that is sometimes associated with massive and active galaxies at high redshift. One idea is that the we are observing cooling radiation from gas that is accreting onto the galaxies. Dijkstra and Loeb endorse this idea, and specifically associate the radiation with cold accretion.

In their model, some fraction of the gravitational energy is converted into heat via weak shocks, which leads to the Ly-a emission. By taking the cold gas fraction that comes from simulations, and putting in a few additional parameters, they are able to calculate the Ly-a emission as a function of halo mass. This leads to the above figure. The curves are different model predictions for how biased a region of space was probed by the survey that led to those data points (are there no good luminosity functions available for LABs?), so I think the comparison of the models and data is illustrative only.

Connecting LBGs to DRGs

2009-02-20T11:46:00.001+01:00

Figure 12 from Stark et al., http://arxiv.org/abs/0902.2907

This paper presents an analysis of Lyman break galaxy candidates at
z=4, 5, and 6 (B, V, and i-dropouts respectively) from the GOODS
fields. In these fields they find 2443 B, 506 V, and 137 i dropouts;
reliable Spitzer data are available for about 35% of them, and they
use the Spitzer data to estimate stellar masses. The above figure
shows the number density of LBGs above log(M)=11 compared to the
Kriek et al. (2008) "red sequence" galaxies at z=2.3 (shown as the
red asterisk). Given the prevalence of massive star-forming galaxies
at higher redshifts, it appears plausible that these could account
for a significant fraction (the authors quote 50%) of the z=2
quiescent galaxy population.

Clustering of DRGs

2009-02-13T12:28:00.002+01:00

Jeremy Tinker, Risa Wechsler and Zheng Zheng

We're right and Ryan is wrong.

A downturn in intergalactic CIV as redshift 6 is approached

2009-02-13T10:31:00.004+01:00

From Ryan-Weber et al. (http://arxiv.org/abs/0902.1991). Caption reads:
"Figure 5. Cosmological mass density of C IV as a function of redshift. The blue squares show the measurements by Songaila (2001), the red triangle is from Pettini et al. (2003), and the green triangle is the value deduced here. All values plotted have been reduced to the ‘737’ cosmology adopted in the present work. Error bars are 1-sigma. While this plot shows the actual values of C Iv measured, they are not strictly comparable because each of the three surveys had a different sensitivity limit. This issue is discussed in detail in the text (section 5)."

The authors claim that the this downturn of metals implies a deficiency of ionizing photons at high-z (that is, there would not be enough photons to keep the universe ionized).

mass build-up of red sequence

2009-01-30T12:21:00.003+01:00

Fig 10. Ruhland et al. http://arxiv.org/abs/0901.4340

They present first measurements of the evolution of the scatter of the cosmic average early-type galaxy color-magnitude relation (CMR) from z=1 to the present day, finding that it is consistent with models in which galaxies are constantly being added to the red sequence through truncation of star formation in blue cloud galaxies.
This fig. shows comparison between observations and a model in which new red sequence galaxies are being constantly added at the rate required to match the observed number density evolution The model predicts the correct CMR scatter and its evolution.

66-page Springel paper

2009-01-30T12:17:00.001+01:00

E pur si muove: Springel, http://arxiv.org/abs/0901.4107

Volker Springel has a new code, and it does neat stuff. Any
theorists in the room want to elaborate?

Another solution to the missing satellite problem

2009-01-30T12:06:00.003+01:00

From Busha et al. (http://arxiv.org/abs/0901.3553)

The authors use the Via Lactea + a cosmological N-Body simulation to model the effects of reionization on the dwarf galaxy population. Previous studies have claimed that reionization did not reduce the number of dwarf galaxies enough. They find they can account for a number as well as the distribution of satellites. One particular way they differ from other studies is that other studies allowed pre-reionization collapsed objects to form stars with a much higher effeciency, while they claim that these objects would have their cold gas photo-evaporated on a short timescale.

The above figure shows the sub-halo population from the simulations (solid black line) and their inferred dwarf distribution (linestyles). The observations are given by the points and the cyan region.

2009-01-30T12:04:00.002+01:00

On the formation of massive galaxies: A simultaneous study of number density, size and intrinsic colour evolution in GOODS
Ignacio Ferreras, Thorsten Lisker, Anna Pasquali, Sadegh Khochfar, Sugata Kaviraj
The evolution of number density, size and intrinsic colour is determined for a volume-limited sample of visually classified early-type galaxies selected from the HST/ACS images of the GOODS North and South fields (version 2). The sample comprises 457 galaxies over 320 arcmin2 with stellar masses above 3E10 Msun in the redshift range 0.4<1.2.

Excess AGN activity in clusters

2009-01-23T11:33:00.001+01:00

from Gilmour, Best, & Almaini, http://arxiv.org/pdf/0901.2810v1

Previous observations have shown that galaxies in clusters tend to exhibit AGN activity more frequently than field galaxies, and so the cluster environment may play a role in the triggering of AGN. However, the radial distribution of AGN in clusters has been a matter of debate: specifically, there have been claims that AGN activity is more prominent in the centers of clusters, while others do not see this.

The above figure shows the excess number of X-ray point sources (relative to blank fields) as a function of cluster-centric radius in 148 X-ray detected clusters. When the central cluster galaxy is disregarded, no excess of sources is seen in the center. Most of the clusters studied by other groups were included in this sample; the authors confirm the previous results (excess of central AGN) with those specific samples, but conclude that those results were due to sample bias and/or cosmic variance.

The Fraction of Quiescent Massive Galaxies in the Early Universe

2009-01-23T11:02:00.003+01:00

A. Fontana, P. Santini, A. Grazian, L. Pentericci, F. Fiore, M. Castellano, E. Giallongo, N. Menci, S. Salimbeni, S. Cristiani, M. Nonino, E. Vanzella
0901.2898

Bottom: Fraction of "Red and Dead" galaxies (defined as SFR/stellar mass <10^-11 yr-1) as a function of redshift. Sample is selected to include only galaxies with stellar masses greater than 7x10^10 solar masses. Filled points are data, lines and shaded region refer to theoretical predictions. Upper panel: Fraction of "Quiescent galaxies" (defined as SFR/stellar mass < age of Universe at redshift of galaxy) for the same mass selected sample.

Missing satellites? No problem!

2009-01-16T12:20:00.002+01:00

Koposov et al., http://arxiv.org/abs/0901.2116

The authors perform a comprehensive analysis of the "missing satellite problem", taking into account realistic selection effects from the SDSS. As the figure above shows, with relatively simple prescriptions for star formation in these halos (e.g. suppression of gas accretion after reionization in halos with v<35 km/s), the data (black bars) actually agree with the fiducial model (green band).

Red Sequence Slope Evolution

2009-01-16T12:09:00.001+01:00

from Stott et al., http://arxiv.org/abs/0901.1227

This paper investigates the evolution of the color-magnitude relation
slope in massive galaxy clusters using two samples: LARCS at z~0.1
and MACS at z~0.5. The observed slope evolution is compared to
predictions from the Bower et al. (2006) semianalytic model (taking
into account AGN feedback and "strangulation). The above figure
shows the rest-frame slope evolution, including the data themselves
(points), a fit to the data (solid line), and the Bower et al.
prediction (dashed line). The discrepancy between the models and
observations is attributed to either the shutdown of star formation
in clusters being stronger than the models predict, or possibly
differential chemical evolution (e.g. faint galaxies have
preferentially higher chemical enrichment rates). Oddly, the
observed-frame red sequence slope matches the models very well;
apparently this is because the observed evolution is dominated by the
K-correction.

Regulation of Black Hole Growth in Low Redshift Galaxies

2008-12-19T12:13:00.003+01:00

This paper by Kauffmann & Heckman discusses the accretion onto central black holes in SDSS. The authors use the OIII line luminosity as a proxy AGN accretion rate, and they infer the black hole mass from the stellar velocity dispersion; thus L[OIII]/M_bh becomes a measure of the Eddington ratio. The upper panel in this figure shows the distribution of Eddington ratios for SDSS galaxies. In the lower panels, the galaxies are split up by the strength of their 4000AA break, which is a measure of galaxy age. It appears that young galaxies have a broad log-normal distribution which is independent of age (it is also independent of M_bh, but that isn't visible in this figure). But older galaxies have an approximate power-law distribution which does depend on age (and also on M_bh).

The authors interpret the log-normal distribution as a reflecting black hole self-regulation, with a negative feedback effect that kicks in at the peak of the distribution (~1% Eddington). But since the distribution doesn't depend on black hole mass or on the star formation in the rest of the galaxy, the feedback must operate only in the immediate vicinity of the black hole. On the other hand, the power-law distribution for the older galaxies does not suggest self-regulation. The authors show that the accretion rate onto the the black hole is roughly proportional to the bulge stellar mass, which is consistent with a scenario in which the black hole is fed by stellar mass loss... however this is a rather speculative conclusion.

The impact of TP-AGB stars on hierarchical galaxy formation models

2008-12-11T12:09:00.003+01:00

From: arXiv:0812.1225 [ps, pdf, other]

Title: The impact of TP-AGB stars on hierarchical galaxy formation models

Authors: Chiara Tonini (1), Claudia Maraston (1), Julien Devriendt (2), Daniel Thomas (1), Joseph Silk (2) ((1) Institute of Cosmology and Gravitation, University of Portsmouth, UK; (2) University of Oxford, UK)

Comments: 5 pages, 4 figures. Submitted to MNRAS Letters

Subjects: Astrophysics (astro-ph)

The authors paint galaxy magnitudes on a semi-analytic model of galaxy formation, comparing two population synthesis packages: Maraston 05 (with 'proper' treatment of TP-AGB stars) and Pegase (without those beasts). here they show the V-K,V color magnitude relation at 4 different redshifts for disks and spheroids. The TP-AGBs are particularly relevant at an age of 1 Gyr in the K band. Mass tot K-band light ratios differ by a factor of ~3, for 1 Gyr SSPs, and less for other wavelength bands and ages (V for example is almost indistinghuishable). This may have big consequences for the fitting of stellar masses on the basis of rest-frame K band photometry, as is often done.

Merger rates at z~3

2008-12-05T12:15:00.000+01:00

from Bluck et al., http://arxiv.org/pdf/0812.0926

The authors use pair counts and galaxy morphologies (CAS) to estimate
merger rates in the GOODS North and South fields. The plot above
shows the merger fraction for galaxies above log(M)=11. At z>3 the
merger rate continues to increase, so the peak of merger activity for
these galaxies must be at higher redshifts. This is in contrast to
lower-mass galaxies (10^10), where the merger peak is seen around
z=2. Thus, the authors conclude that high-mass galaxies undergo
major mergers at higher redshifts than lower-mass galaxies.

The growth of supermassive black holes in pseudo-bulges, classical bulges, and elliptical galaxies

2008-11-14T11:28:00.003+01:00

Last week Jo talked about a paper by Greene et al. that showed (if I remember correctly) that galaxies without classical bulges also contain black holes, and that presented evidence that black holes in low-mass bulges don't follow the normal relationship between bulge mass and black hole mass. A possible explanation for this was that the low-mass bulges tend to be pseudo-bulges, and that pseudo-bulges have different properties than standard bulges.

Now Gadotti & Kauffmann have presented an analysis of SDSS data that appears to support this conclusion. This plot shows the bulge mass vs. velocity dispersion for ellipticals, classical bulges, and pseudo-bulges. The ellipticals follow a tight relation, and the classical bulges also follow a fairly tight relation but with an offset. However the pseudo-bulges don't seem to follow much of a relation, tend to have significantly lower masses than ellipticals/classical bulges at a fixed velocity dispersion.

If pseduo-bulges don't follow the same M_bulge-sigma relation as other galaxies, then they can't follow both the standard M_bh-M_bulge relation and the M_bh-sigma relation at the same time. Perhaps pseudo-bulges follow only one of these relations (as suggested by the Greene et al. paper), or maybe they follow neither.

One possible explanation mentioned by the authors for the observation that pseudo-bulges don't follow the same M_bulge-sigma relation is that bars (which may be so small as to be undetected) artificially enhance the observed sigma. Another is that pseudo-bulges aren't relaxed, so the virial theorem doesn't apply.

2008-10-30T18:00:00.003+01:00

DISSECTING THE RED SEQUENCE—I. STAR FORMATION HISTORIES OF QUIESCENT GALAXIES: THE
COLOR-MAGNITUDE VS. THE COLOR-SIGMA RELATION

Genevieve J. Graves, S. M. Faber, & Ricardo P. Schiavon

From DR4 SDSS data with the NYU-VAGC, the authors select quiescent (i.e. emission line-free) galaxies. These lie on the red sequence. They measure luminosities, colors, velocity dispersion and a few element abundances (mainly Fe abundance and alpha-enrichment). In a plot that I don't show here, it shows that at fixed velocity dispersion there is no relation between luminosity and color (luminosities vary more than color, and contours of number density are largely horizontal). The red sequence is inclined, because higher sigma galaxies, have higher luminosities and are redder. Adding up different sigma bins results in the red sequence as we know it.

To investigate this further, the authors bin in L-sigma-color space and stack all spectra in a bin together to measure abundances and age. Here I will concentrate on age. In the plot you see six panels in the luminosity color plane, for six bins in velocity dispersion (these show no L-color relation!). The color coding is luminosity weighted mean age.

As you can see, low-sigma galaxies are younger and have a bigger spread in ages than luminous, high-sigma galaxies. Also, the variation in age is perpendicular to the red sequence, i.e. the width is set by the age distribution of the galaxies.

Quiescent galaxies are a multi-parameter family. Age (and Fe/H and alpha/Fe) all increase with sigma, and vary at fixed sigma depending on L. Age also varies as a function of color, at fixed sigma: brighter galaxies have lower age at fixed sigma and are bluer.

Evidence for Merger-Driven Activity in the Clustering of High Redshift Quasars

2008-10-24T11:17:00.003+02:00

Recently Shen et al. (2007) found that high-z quasars from SDSS are very strongly clustered, with a bias of ~14 at z=4. White et al. (2008) used this result to show that there must be an extremely tight relation between quasar luminosity and halo mass, with an upper limit to the scatter of 0.3 dex. The basic reasoning behind this conclusion is that, if there were a larger scatter, then many quasars in (very abundant) low-mass halos would have high enough luminosity to make it into Shen's sample, however the observed number density of quasars is too low to allow for this. Wyithe & Loeb say that such a tight scatter is difficult to believe since the scatter in the relationship between black hole mass and bulge velocity dispersion is also 0.3 dex, and one might expect that this relationship is tighter and more direct then the relationship between halo mass and quasar luminosity.

Now Wyithe & Loeb have revisited this issue, using a somewhat more flexible model than was used by White et al. For instance, White et al. assumed that quasar luminosity is proportional to halo mass, whereas physical arguments suggest that it should be proportional to halo mass to a higher power. Also, Wyithe and Loeb allow for an arbitrary boost in the clustering of halos that host quasars. Such a boost might be expected if those halos have special properties, for instance if they have just merged.

This figure shows the joint likelihood distributions of various parameters in the Wyithe & Loeb model. I won't bother to explain all of the parameters, so just look at the upper right plot. This shows contours of F (the amount that the bias is boosted by) vs. gamma (the slope of the halo mass vs. quasar luminosity relation). Models where F=1 are highly disfavored. This suggests that you can't explain the observed quasar number density and clustering using a standard clustering model, but that some other ingredient must come into play.

This conclusion is related to some other recent results, as I mentioned here, however those results may be subject to systematic observational uncertainties. Perhaps the Shen et al. measurement is on firmer ground, but I haven't looked at that paper in detail. But it does seem that the very high clustering measurement is in contrast with the measurement presented by Adelberger & Steidel (2005).

2008-10-24T10:49:00.001+02:00

Ok, I'll kick it off this week. This is a plot from Shen et al (2008, arXiv:0810.4144) who study the correlation of QSOs in the sky using the SDSS DR5. They look how clustering depends on luminosity, black hole mass, colour and radio loudness. What they find is perhaps a bit surprising but not entirely new - there is virtually no dependence on any of these parameters, except radio loudness. So - don't go around expecting a QSO to necessarily live in a massive halo, at least not at z<2.5

Red Nugget Watch

2008-10-17T11:38:00.001+02:00

from Saracco, Longhetti, & Andreon, http://arxiv.org/abs/0810.2795

This paper presents another analysis of the sizes and surface densities of early-type galaxies at z=1-2; here they use a sample of 32 spectroscopically-confirmed galaxies with a mean redshift of 1.45 from several different surveys. Masses and ages are determined via SED fitting to the photometry. As has been reported before, these galaxies lie well off the z=0 size-luminosity relation. By evolving the galaxies (assuming pure luminosity evolution) from their measured redshifts to the present, the authors find that some galaxies would actually evolve to the z=0 relation in this manner, while some would not. These galaxies appear to be "young" and "old" respectively, and the authors appear to claim a bimodal age distribution in ETGs at this redshift, with typical ages ~1 Gyr and 3.5 Gyr. Young ETGs follow the local size-mass relation, old ETGs do not.

The authors conclude that the young objects have more or less completed their evolution (except for luminosity evolution), while the old galaxies still need some process to increase their effective radii. Dry merging cannot do this because it would create too many high-mass galaxies, so some other process must be at work. Much of this rests on the assumption that the relative ages can be accurately determined through photometry, of course.

The PN.S Elliptical Galaxy Survey: the dark matter in NGC 4494

2008-10-10T12:01:00.003+02:00

The PN.S Elliptical Galaxy Survey is an ongoing survey to detect and measure the kinematics of planetary nebulae (PNe) in nearby early-type galaxies. Here they present 255 PNe measurements in the elliptical NGC 4494 out to 7 effective radii (Re). They construct mass models, where they include dark matter haloes to explain the observed kinematics at large radii. This plots shows the dark matter fraction they find in this galaxy and other galaxies in their sample, compared to results from numerical simulations. Overall, the dark matter fraction is lower than predicted in simulations, especially at smaller radii. This indicates a mismatch between observations and theory, with intermediate-luminosity galaxies having low concentration haloes.

From Napolitano et al., from http://arxiv.org/abs/0810.1291

Reconstruction of a z=3.07 lensed galaxy

2008-10-10T11:56:00.001+02:00

from Stark et al., http://arxiv.org/abs/0810.1471

This Nature paper describes integral field (OSIRIS on Keck) observations of a strongly lensed z=3.07 Lyman break galaxy. With AO corrections and the high magnification, these observations provide an effective physical resolution of 150 kpc. This figure shows (a) the reconstructed HST image with the lensing caustic overlaid, (b) [OIII] and Hbeta emission (bluescale and contours respectively), (c) velocity field and best-fit disk model, (d) [OIII] velocity dispersion, (e-h) 1-d profiles of the left panels taken along the "slit" shown in panel (d). The velocity field is well-fit by a disk model, so the authors conclude that this is a disk galaxy with v_r=67 km/s and M=2e9 Msun. However, the central velocity dispersion is large (v/sigma=1.2), so it's most likely still at an early stage of formation.

all MW halos have the same mass!

2008-10-10T02:23:00.001+02:00

All the satelites of our galaxy have aroundabout the same mass enclosed within a fixed radius (see Strigari et al., arXiv:0808.3772). Here, the authors have taken a N-body sim + SAM to see whether or not this falls out of current models. And indeed it does! The black points show their model results; the red points are from Strigari et al.

Another interesting sidenote: of the 2000ish subhalos that the authors tracked, only 51 became fully fledged satelite galaxies. This is still twice as many as are observed, but they reckon that optical selection effects can account for this. (Which you can kinda see from the plot.) This is essentially because they have completely supressed gas cooling in subhaloes with virial temperatures below 10^4 K.

Evidence for a Collision Between M86 and NGC 4438 and Implications for Collisional ISM Heating of Ellipticals

2008-10-07T14:20:00.001+02:00

Kenney et al.

This is a color gri from SDSS, overlayed with narrowband Halpha+NII images (visible as the red and green filaments). The giant elliptical on the right is M86, which appears to be the brightest galaxy in a group or sub-cluster that is merging with Virgo. The galaxy on the left is NGC 4438 (also Arp 120), a highly-disturbed spiral. The red filaments are Halpha+NII emission that appear to link the two galaxies, suggesting that they have undergone a high-speed collision. The green filaments are Halpha+NII emission at a higher recessional velocity; it is not clear whether the galaxy in the lower right and the associated line emission are involved in this interaction.

NGC 4438 is very HI-deficient for a spiral galaxy of it's size. If it lost most of it's HI during the collision, then it is expected that a significant fraction of the kinetic energy of that gas went into heating the ISM of M86. This heating would be enough to prevent gas from cooling and forming stars in M86, possibly obviating the need for radio-mode AGN feedback. Thus this interacting system may be a nice example of the "gravitational quenching" mechanism discussed by Dekel & Birnboim (2008).

Time to go into finance?

2008-09-26T11:39:00.001+02:00

Conroy, Gunn, & White, http://arxiv.org/abs/0809.4261

This paper explores the effects of stellar evolution uncertainties
(particularly the properties of thermally pulsating AGB stars, but
also metallicity and the IMF) on quantities derived through stellar
population modeling like age, mass, and star-formation rate. From
computing expected colors of LMC star clusters the authors conclude
that the temperature and luminosity of the TP-AGB phase could vary by
as much as +-0.2 dex and +-0.4 dex respectively, and so they allow
these parameters to vary in their stellar population fitting. The
plot above shows 68% and 95% likelihood contours for the derived
properties for a bright z~2 quiescent galaxy, with probability
distributions (blue line: AGB star uncertainties included; black: not
included) in the top panel. Interestingly, the degeneracies between
the AGB parameters and derived quantities are weak at best, and the
uncertainties don't seem to increase much (blue vs. black curves).

Now you see it, now you don't

2008-09-12T12:05:00.003+02:00

Barbary et al (2008, http://uk.arxiv.org/abs/0809.1648) have discovered an (apparently) new class of optical transient objects. The spectrum is a mystery, the source brightened by >5-6 magnitudes over 100 days and no host galaxy can be seen. Might be Galactic, might be extra-galactic. If the latter, the most reasonable estimate of redshift gives a peak luminosity of -22.1 - close to the brightest SNe seen.

Do Sub-mm Galaxies Really Trace The Most Massive Dark Matter Halos?

2008-09-12T11:57:00.003+02:00

Chapman et al.

This paper presents evidence of a strong over-density of sub-mm galaxies at z=1.99 in GOODS-N. The interesting thing about it is that there is also an over-density of the more typical blue star-forming galaxies at the same redshift, but that the density contrast for the blue galaxies is much weaker than for the sub-mm galaxies.

The authors suggest that this is a cluster in the first stages of formation. The strength of the over-density of sub-mm galaxies is due to the numerous ongoing mergers, and thus it is not representative of the overall matter over-density, which would be much weaker. The fact that the masses and SFRs of the blue star-forming galaxies in the redshift spike is similar to the values for galaxies outside of the spike supports this argument, since you would expect most galaxies in a cluster to be older and more massive (even at these high redshifts).

The biggest caveat that the authors note is that there may be a large population of quiescent galaxies in this redshift spike that have not been observed spectroscopically. If this were the case, the true matter over-density would be closer to the over-density of sub-mm galaxies, and the merger argument wouldn't be necessary. The same caveat applies for blue star-forming galaxies that don't have spectroscopic redshifts. So without having a good idea of the statistical significance of these results, I would say that the main conclusion is a bit of a stretch... although not any more so than some other recent claims in the literature.

Obscured Star Formation in Abell 901/902

2008-09-12T11:46:00.004+02:00

The authors investigate the amount of obscured star-formation as a function of environment. They find that ~40% of the star forming galaxies has red optical colors at intermediate and high densities. This suggests that environmental interactions trigger a phase of obscured star formation before complete quenching.

DOG fight

2008-09-05T11:25:00.001+02:00

from Pope et al., arXiv:0808.2816. Dust-obscured galaxies (DOGs)
have been recently defined as objects with extremely red R-[24um]
colors. Another recent paper by Fiore et al. studied the X-ray
properties of a similarly-selected set of galaxies and found that
they are consistent with being Compton-thick AGN. The DOG people
claimed that these things were primarily starbursts, and so they
launch a counteroffensive with the above figure. While most bright
DOGs show a strong 8um excess (and are therefore likely powered by
AGN), most of the overall sample seems to be dominated by star formation.

All MW satellites have the same mass?

2008-08-29T10:54:00.002+02:00

from Strigari et al., arXiv:0808.3772

Line-of-sight velocity measurements were used to derive masses within the innermost 300pc of 18 Milky Way dwarf satellites. The above figure is self-explanatory, and surprising: over 5 orders of magnitude in luminosity, there's almost no change in the dark matter+stellar mass. They mention several possible explanations, including a sharp cutoff in star formation efficiency below this halo mass, a characteristic formation time (in CDM models) around the epoch of reionization, or dark matter temperature of >1keV (in WDM models).

Contradiction between strong lensing statistics and a feedback solution to the cusp/core problem

2008-08-22T11:43:00.003+02:00

Title: Contradiction between strong lensing statistics and a feedback solution to the cusp/core problem
Authors: Da-Ming Chen, Stacy McGaugh
arXiv:0808.0225

Abstract:
Standard cosmology has many successes on large scales, but faces some fundamental difficulties on small, galactic scales. One such difficulty is the cusp/core problem. High resolution observations of the rotation curves for dark matter dominated low surface brightness (LSB) galaxies imply that galactic dark matter halos have a density profile with a flat central core, whereas N-body structure formation simulations predict a divergent (cuspy) density profile at the center. It has been proposed that this problem can be resolved by stellar feedback driving turbulent gas motion that erases the initial cusp. However, strong gravitational lensing prefers a cuspy density profile for galactic halos. In this paper, we use the most recent high resolution observations of the rotation curves of LSB galaxies to fit the core size as a function of halo mass, and compare the resultant lensing probability to the observational results for the well defined combined sample of the Cosmic Lens All-Sky Survey (CLASS) and Jodrell Bank/Very Large Array Astrometric Survey (JVAS). The lensing probabilities based on such density profiles are too low to match the observed lensing in CLASS/JVAS. High baryon densities in the galaxies that dominate the lensing statistics can reconcile this discrepancy, but only if they steepen the mass profile rather than making it more shallow. The result is contradictory demands upon the effects of baryons on the central mass profiles of galaxies.

Constraints on high-z disk formation

2008-08-15T10:42:00.001+02:00

figure 1 from Robertson & Bullock (arXiv:0808.1100)

The authors investigate the claim by Genzel et al. (2006) that the
z=2.4 "disk" galaxy BzK-15504 formed very rapidly and early on (with
a correspondingly rapid accretion of mass), but shows no evidence of
a merger (because its velocity asymmetry is low). Using their
simulations of gas-rich disk mergers and taking into account noise
and PSFs, the authors figure out how their merger remnants would
appear viewed through SINFONI. In all four of their simulations
(each with different initial configurations), galaxies that would be
observationally classified as "disks" appear 100-150Myr after the
merger. The above figure shows one particular simulation that they
claim matches the kinematic properties of BzK-15504 (shown below for
comparison, from Genzel et al. 2006) remarkably well.

Observations of the Gas Reservoir around a Star Forming Galaxy in the Early Universe

2008-08-14T14:11:00.003+02:00

Frye et al., http://arxiv.org/abs/0808.0921

The figure shows a high-S/N spectrum of a z=4.9 starburst galaxy; this kind of S/N is only possible because of a long exposure time (14hrs on an 8m telescope) and because the flux is boosted by a factor of 10 due to gravitational lensing by a foreground cluster. The most interesting feature is the broad Gunn-Peterson trough blueward of the bright Lya emission line.

Since the optical depth in this trough is significantly larger than observed at the same wavelengths for random sight-lines through the IGM (the universe was already reionized at this redshift), the authors conclude that we are seeing direct evidence of a "cosmic web" of gas that surrounds galaxies and feeds their growth.

Another possibility is that the neutral gas is outflowing material from the galaxy itself, however the authors discount this explanation since stellar population modeling suggests that the galaxy is too young to have driven such large amounts of gas outwards. Also, typical outflow velocities are not large enough to explain the broad trough, even for outflows that are powered by AGN.

This is a neat observation, but I'm not sure how much we can infer from a single object. Unfortunately, a galaxy at this redshift has to be strongly lensed to be bright enough for this sort of analysis, so there isn't much hope of obtaining a large sample in the near future. Of course QSOs are also bright enough, but they tend to ionize most of the hydrogen in their immediate vicinity.

Size evolution from z=1 to present

2008-08-08T12:01:00.001+02:00

figure 7 of van der Wel et al., arXiv:0808.0077

In this paper, the authors find that early-type galaxies grow larger
by a factor of ~2 from z=1 to z=0, consistent with previous studies.
This figure shows the ratio of sizes at the two redshifts from this
work and others, and compares it to the Khochfar & Silk (2006)
semianalytic model. The observed size evolution between z=1-0 is
much steeper than predicted by the model, which is based on the idea
that (cold)-gas-rich mergers at high redshift produce smaller
galaxies than the gas-poor mergers at low redshift.

Probably the most important difference between this and previous
studies is that masses here are calculated using dynamical, rather
than photometric, measurements. Thus, the mass (and hence surface
density) estimates here should be less prone to systematic effects.
Nonetheless, the offset in the size-mass and size-surface density
relations are consistent with the photometric studies. This seems to
imply that any systematic effects are small compared to the observed
size evolution.

The Millennium Simulation compared to z~2 galaxies

2008-08-08T11:14:00.004+02:00

The Millennium Simulation compared to z~2 galaxies
Genel et al.
0808.0194

The authors use the Millennium Simulation to extract merger fractions and
mass accretion rates. They find that the accretion rates are sufficient to
account for the high star formation rates observed in z~2 UV-optically selected disks.

(not in figure) When following the fate of these disks and submm galaxies, they find that subsequent mergers are not frequent enough to either convert all disks into elliptical galaxies at z~0, or transform all submm galaxies to massive cluster ellipticals at z~0. They conclude that secular and internal evolution must play an important role in the evolution of these z~2 galaxies

2008-08-07T17:58:00.002+02:00

Disc instabilities and semi-analytic modelling of galaxy formation
E. Athanassoula

This paper points out that the method to form spheroids in semi-analytic models of galaxy formation are wrong. A criterium, based on bar instability for given disk parameters is often used in these models and either the whole disk, or a fraction of the disk than suddenly transforms into a spheroid. This is a necessary ingredient to match the observed near-IR luminosity functions.

In reality, this criterium does not hold. It was derived from 2D Nbody simulation long ago. If you take into account that the halo is non-static and that there are random motions in the disk and in the halo, then the situation is different: disks are much more stable, and form at best small pseudo-bulges. A simple creterium for bulge formation from disk instability is not easily possible, and should not be used in semi-analytic models.

On the SFR-brightest cluster relation: estimating the peak SFR in post-merger galaxies

2008-08-01T11:32:00.004+02:00

Nate Bastian: astro-ph/0807.4687

He does Monte Carlo simulations and comparisons to observations of star cluster formation in galaxies. It appears thet there is quite a tight relation between the magnitude of the brightest young cluster present, and the current SFR, because the brightest cluster often is young (<15Myr) and the mass of the most massive cluster is determined by the number of clusters formed, which is strongly related to the SFR. In the figure, the colored lines are lines of different cluster mass functions (schechter with some M*. ). The gamma indicates the factor between star formation rate and cluster formation rate, i.e. they find that CFR = 0.08 SFR, which is sort of low. This SFR indicator agrees well with other indicators.

Using the fading of clusters, an old cluster that is very bright can indicate a burst in SFR some time ago, and its luminosity and age give an estimate of the peak SFR of that galaxy (or galaxy merger). Here cluster disruption effects need to be taken into account (which is difficult).

An Imprint of Super-Structures on the Microwave Background due to the Integrated Sachs-Wolfe Effect

2008-07-31T22:30:00.005+02:00

Benjamin R. Granett, Mark C. Neyrinck, István Szapudi (IfA, Hawaii)

We measure hot and cold spots on the microwave background associated with supercluster and supervoid structures identified in the Sloan Digital Sky Survey Luminous Red Galaxy catalog. The structures give a compelling visual imprint, with a mean temperature deviation of 9.6 +/- 2.2 microK, i.e. above 4 sigma. We interpret this as a detection of the late-time Integrated Sachs-Wolfe (ISW) effect, in which cosmic acceleration from dark energy causes gravitational potentials to decay, heating or cooling photons passing through density crests or troughs. In a flat universe, the linear ISW effect is a direct signal of dark energy.

FIG. 1.— Stacked regions on the CMB corresponding to supervoid and supercluster structures identified in the SDSS LRG catalog. We averaged CMB cut-outs around 50 supervoids (left) and 50 superclusters (center), and the combined sample (right). The cut-outs are rotated, to align each structure’s major axis with the vertical direction. Our statistical analysis uses the raw images, but for this figure we smooth them with a Gaussian kernel with FWHM 1.4. Hot and cold spots appear in the cluster and void stacks, respectively, with a characteristic radius of 4, corresponding to spatial scales of 100 M pc/h inner circle (4 radius) and equal-area outer ring mark the extent of the compensated filter used in our analysis. Given the uncertainty in void and cluster orientations, small-scale features should be interpreted cautiously.

2008-07-24T09:53:00.004+02:00

From the paper 'Red Nuggets at z ∼ 1.5: Compact passive galaxies and the
formation of the Kormendy Relation' by Damjanov et al (0807.1744) I chose these two plots.

We had discussions before about these tiny galaxies that Mariska and Pieter investigated. These authors do sort of the same job, but at slightly lower redshift (1.5-ish). The left plot shows the effective radius - stellar mass plane, with the dots and contours being local SDSS red galaxies. The bigger points with error bars are their (and some other) red galaxies at higher redshift, which appear to small. I show this plot, because the arrows indicate the approximate track of evolution due to three different processes: dry mergers, pure stellar mass growth without changing size and adiabatic expansion (stellar mass loss makes the systems less bound). All three processes seem incapable of transforming the galaxies towards the low - z counterparts.

The right plot shows the galaxies in the stellar mass density - effective radius plane (Kormendy relation). Here they are all on the same trend, with the high redshift galaxies smaller and denser than their local red SDSS partners. Color coding here is redshift, which appears to hint at some evolution: the higher the redshift of the galaxy, the smaller and denser it is. The main part of the evolution takes place at 1.1 < z < 1.5.

What's wrong with this picture?

2008-07-03T15:10:00.004+02:00

(or, at least, highly suspicious--and why?):

from Morioka et al., arXiv:0807.0101, PASJ in press. Black dots are H-alpha emitting galaxy candidates (via a narrowband filter selection), grey regions are masked-out bright stars. The authors use this sample to compute the clustering and luminosity function of star-forming galaxies at z=0.24, and note that the clustering in this field is stronger than in the COSMOS field. First one to post the right answer in comments wins a beer at the next borrel.

It's the same!

2008-06-25T12:10:00.005+02:00

From arXiv:0806.3278

Title: Correlations between MIR, FIR, H$\alpha$, and FUV Luminosities for SWIRE galaxies

Authors: Yi-Nan Zhu, Hong Wu, Chen Cao, Hai-Ning Li

The figure shows the correlation between observed H-alpha + 24 micro (y-axis, left-hand panel) and H-alpha + 8 micro (y-axis, right-hand panel) luminosities with the extinction-corrected H-alpha luminosities (x-axis) for star-forming galaxies selected from the Spitzer-SWIRE fields.

Filled circles: Normal galaxies
Open circles: Dwarf galaxies
Lines: Best nonlinear (solid) and linear (dotted) fit

SXDF SMGs & BzKs

2008-06-06T12:21:00.001+02:00

from Takagi et al., http://arxiv.org/abs/0806.0888

The authors attempt to investigate whether submillimeter galaxies can be
identified by simple color cuts. The answer is: probably not, with the
resolution of current submm instruments.

Spectroscopic Confirmation Of An Extreme Starburst At Redshift 4.547

2008-06-06T12:20:00.002+02:00

Capak et al., http://arxiv.org/abs/0806.0657

This is a set of images of an extreme object at z=4.5 in the COSMOS survey. It is the most distant mm source not associated with an optically bright quasar. The rest-frame UV and Lya imaging (the first five panels on the left) show emission near the lower left of the panes, although no emission is detected in the B-band because it falls blueward of the Lyman break. At longer wavelengths, the emission shifts to the upper left; the panel on the far right shows radio contours.

The estimated star formation rate is 1000-4000 Msun/yr, based on several indicators. The authors argue against significant AGN activity -- which would mean this SFR could be a severe overestimate -- because there is no xray detection and because an optical spectrum shows no hint of an AGN. But, as the authors note, an AGN could lie outside the optical slit. In fact I think this a fairly likely explanation since the authors placed the slit on the center of the UV emission (lower left), whereas an AGN would be expected to be associated with the longer wavelengths (upper right), which is where the most of the stellar mass and radio activity is.

Evidence for a Stellar-Dominated UV Background and Against a Decline of Cosmic Star Formation Beyond z~3

2008-06-06T12:13:00.002+02:00

From Faucher-Giguere et al (http://arxiv.org/abs/0806.0372). The authors use the Lyman-alpha forest opacity to estimate the photoionizing background at 2 < z < 4.2. After subtracting the contribution from AGNs, they suggest that stellar sources are dominant (and thus may be responsible for reionization). Perhaps more remarkably, they determine a cosmic star formation rate that is flat, at odds with the well accepted Hopkins and Beacom (2006) curve.

Radio jet duty cycles in nearby galaxies

2008-05-30T12:07:00.002+02:00

from Shabala et al., arXiv:0805.4152

This figure shows the fractile distribution of radio source ages in four different stellar mass bins. There appears to be a strong trend whereby more massive galaxies host older radio sources, suggesting that the "on phase" of radio activity lasts longer for these galaxies. The on-time and the gas cooling rate show the same dependence on stellar mass, suggesting that the two are probably linked (i.e. the availability of fuel governs whether an AGN is on or off). Elsewhere in the paper the authors argue that higher-mass galaxies also exhibit more powerful and frequent radio jets. If true, this may be further evidence for a link between radio AGN and the shutdown of star formation.

Galaxy Size Problem at z=3: Simulated Galaxies Are Too Small

2008-05-23T11:37:00.002+02:00

(http://arxiv.org/abs/0805.3150 by M.K. Ryan Joung (Princeton), Renyue Cen (Princeton), Greg Bryan (Columbia))

The authors run zoom simulations and find the simulated galaxies are too small to match the observations after corrections (blue line, bottom panel). I suspect that their feedback isn't strong enough to disrupt the inner star formation though.

Red Galaxy Growth and the Halo Occupation Distribution

2008-04-18T12:25:00.002+02:00

Michael J. I. Brown et al astroph 0804.2293

2008-04-18T11:58:00.003+02:00

This is from the paper "The Halpha Galaxy Survey V. The star formation history of late type galaxies", by Phil James et al. astro-ph/0804.2167

They have SFRs from Halpha, stellar masses from K and R band photometry (all with the 1m Kapteyn Telescope) and gas masses from Westerbork neutral hydrogen observations for local late type field galaxies. Here they plot the star formation timescale (Mstar/SFR) and gas depletion timescale (Mgas/SFR) for galaxies as a function of mass and type. The dashed lines indicate the age of the universe. The fact that the star formation timescale for low mass/late type galaxies is similar to the age of the universe and their gas depletion time is much longer, whereas it is reversed for the high mass/earlier type galaxies is used as an argument that the star formation history of very late type galaxies is constant over the age of the universe and the stellar mass gradually builds up, an that for more bulgy galaxies the bulk of the star formation happens in short bursts of high SFR.

The possibility of having a higher SFR in the past is not mentioned...

Predicted OVI-galaxy cross-correlation

2008-04-18T11:04:00.002+02:00

Figure 2 of Ganguly, Cen, Fang, Sembach, http://arxiv.org/abs/0803.4199

The authors use a CDM simulation which includes IGM metal enrichment from superwinds to predict the galaxy-OVI absorber cross-correlation at low redshifts; or, as in the figure above, the fraction of galaxies with an OVI absorber within a given distance, at different galaxy luminosities, absorber strengths, and projected absorber-galaxy separations. They find that the correlation length depends strongly on galaxy luminosity (with faint, low-mass galaxies having more nearby absorbers on average) but not on absorber strength. Only ~15% of OVI absorbers come from >L* galaxies, implying that IGM enrichment may be predominantly due to many faint sources rather than a few bright ones.

They note also that these results are somewhat preliminary and their simulation resolution may cause problems for the lowest-mass galaxies, but these will be a valuable starting point for comparison to upcoming large COS surveys.

The energy output of the Universe from 0.1 micron to 1000 micron

2008-04-11T12:16:00.002+02:00

From http://arxiv.org/abs/0803.4164.

Evolution of the field galaxy pair fraction

2008-04-11T12:12:00.002+02:00

Figure 13 of Hsieh et al., http://arxiv.org/abs/0804.1604

This plot shows the average number of galaxy companions as a function of redshift for different pair separations. With increasing separations, the evolution of the pair fraction decreases. Assuming these pairs represent early-stage mergers, this may imply that the infall/merging timescales are changing with redshift: at high redshift it takes longer for a galaxy to finish merging (i.e. go from 20 kpc to 0 kpc) than at low redshift, relative to the inital (r=150 to 50 kpc) infall. The authors suggest that such a change in timescale may be due to dark matter halos at lower redshift being more concentrated: since the density is lower in the outskirts of highly-concentrated halos, the dynamical friction timescale at large radii is longer, and therefore one might expect merging galaxies to spend more time at larger radii (thus increasing the large-separation pair fraction) at lower redshifts.

The MBH-Sigma relation in the last six billion years

2008-04-11T11:32:00.002+02:00

Figure 2 from Woo et al., astro-ph 0804.0235

The M_BH-sigma relation of active galaxies.
Left panel: local Seyferts with sigma from Greene & Ho (2006) and our own M_BH estimates consistently calibrated with our estimates for distant samples (black circles); local Seyferts with M_BH, measured via reverberation mapping (Onken et al. 2004; magenta circles)
Right panel: new measurements at z=0.57 (red stars); Seyfert galaxies at z=0.36 from our earlier work (blue circles). The local relationship of quiescent galaxies (Tremaine et al. 2002; black points) are shown for comparison as a solid (Tremaine et al. 2002) and dashed (Ferrarese & Ford 2005) line.