An x-ray detected cluster at z=1.62

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

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.

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

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

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.

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

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

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

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

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

Tacconi et al.

The growth of massive galaxies since z=2

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

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

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

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.

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

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

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.

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

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.