Friday, 19 December 2008

Regulation of Black Hole Growth in Low Redshift Galaxies

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.

Thursday, 11 December 2008

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

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.

Friday, 5 December 2008

Merger rates at z~3

from Bluck et al.,

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.

Friday, 14 November 2008

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

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.

Thursday, 30 October 2008


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.

Friday, 24 October 2008

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

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).

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

Friday, 17 October 2008

Red Nugget Watch

from Saracco, Longhetti, & Andreon,

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.

Friday, 10 October 2008

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

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

Reconstruction of a z=3.07 lensed galaxy

from Stark et al.,

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!

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.

Tuesday, 7 October 2008

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

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).

Friday, 26 September 2008

Time to go into finance?

Conroy, Gunn, & White,

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).

Friday, 12 September 2008

Now you see it, now you don't

Barbary et al (2008, 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?

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

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.

Friday, 5 September 2008

DOG fight

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.

Friday, 29 August 2008

All MW satellites have the same mass?

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).

Friday, 22 August 2008

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

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

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.

Friday, 15 August 2008

Constraints on high-z disk formation

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.

Thursday, 14 August 2008

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

Frye et al.,

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.

Friday, 8 August 2008

Size evolution from z=1 to present

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

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

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

Thursday, 7 August 2008

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.

Friday, 1 August 2008

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

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).

Thursday, 31 July 2008

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

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.

Thursday, 24 July 2008

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.

Thursday, 3 July 2008

What's wrong with this picture?

(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.

Wednesday, 25 June 2008

It's the same!

From arXiv:0806.3278

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

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

Friday, 6 June 2008


from Takagi et al.,

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

Capak et al.,

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

From Faucher-Giguere et al ( 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.

Friday, 30 May 2008

Radio jet duty cycles in nearby galaxies

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.

Friday, 23 May 2008

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

( 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.

Friday, 18 April 2008

Red Galaxy Growth and the Halo Occupation Distribution

Michael J. I. Brown et al astroph 0804.2293

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

Figure 2 of Ganguly, Cen, Fang, Sembach,

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.

Friday, 11 April 2008

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


Evolution of the field galaxy pair fraction

Figure 13 of Hsieh et al.,

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

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.

Friday, 28 March 2008

From The author postulates that a natural explanation for the observed metallicity gradient in DLAs can be explained via dense, metal poor disks and less dense metal rich outflows.

Star formation in BCGs

(Figure 10 of O'Dea et al.,

Upper limits on the mass deposition rate (derived from X-ray emission measurements) vs. star formation rate for brightest cluster galaxies. Filled circles denote the maximum mass deposition rate (assuming no gas heating), while open circles are the rates derived from detailed cooling-flow model fits.

This paper discusses results from a Spitzer survey of 62 brightest cluster galaxies that exhibit optical line emission, specifically the ~50% of those galaxies that show excess infrared emission. While four of these appear to be dominated by AGN, they claim that the remainder show evidence for star formation (from L_IR, CO measurements, and Halpha luminosity). The above figure then appears to show a strong correlation between mass deposition from the ICM and star formation, and therefore such cooling may be the fuel source for the SF activity in these galaxies. Furthermore, the MDR exceeds the SFR, indicating that some reheating mechanism may be at work. It should be noted that all the points in this figure are formally upper limits, so in fact the cooling rate could be entirely consistent with the SFR.

Friday, 14 March 2008

arXiv:0803.1489 [ps, pdf, other]
Title: The contribution of star formation and merging to stellar mass buildup in galaxies
Authors: Niv Drory (1), Marcelo Alvarez (2) ((1) MPE, Garching, Germany, (2) KIPAC-Stanford, USA)
Comments: Accepted for publication in ApJ
Subjects: Astrophysics (astro-ph)
The authors take data from the FORS Deep Field (FDF), 5500 galaxies with redshifts between 0 and 5. They obtain stellar masses and SFRs as well as photo-z's. By integrating the SFR over time they calculate, as a function of mass, the change in number density of galaxies of that mass due to star formation. They compare that to the actual change of the mass function to obtain the contribution that is from anything other than star formation, being mainly merging and accretion (assuming that tidal stripping and so on are negligable). They call this the assembly rate and that is what is shown in the figure (the y-axis normalized to have the relative change), as a function of z and mass (in both plots). See the paper for selection effects...

Monday, 10 March 2008

Are SNIa a two-component family?

In astro-ph/0803.1130, Dahlen, Strolger & Riess report on their HST survey for high redshift SN Ia. Their results strengthen the argument that the SNIa rate drops at high redshift. The immediate interpretation of this is that SN Ia only start after some long delay. Their favourite model has a delay time of 3.7 Gyr and is shown as the solid line in the Figure. The solid points in the Figure shows the observed SN Ia rate.

Now the problem is that a number of groups have recently argued, fairly convincingly, that SN Ia at lower redshift clearly fall into two categories: A group of delayed explosions and a group of "prompt" explosions - ala SN II. Three such models are shown as non-solid lines in the Figure and the match to the observations fails to excite :) What gives? The authors argue that dust obscured star formation might explain the differences, but it is not settled.

Friday, 7 March 2008

The evolution of submillimetre galaxies: two populations and a redshift cut-off

Wall, Pope, & Scott 2008, MNRAS, 383, 435

These authors find reasonably secure counterparts for 35/38 SCUBA sub-mm sources in GOODS-N. This plot shows the relationship between redshift (combination of spectroscopic and photometric redshifts) and rest-frame 850 micron luminosity (which is also illustrated by the size of the circular plotting symbols). The black symbols are those below the median luminosity, and the blue symbols are above.  The curves show the detection limits; since the noise varies strongly over the SCUBA map, a different completeness curve is shown for the location on the map of each detected galaxy.  The fact that these curves flatten at high redshift is due to the negative K-correction.

One notes an apparent lack of high-luminosity sources at z<~1.5 and of low-luminosity sources at z>~2.5.  This is interpreted as an aspect of the generic downsizing trend of star-formation.  The authors split the sample in half by luminosity, and model the evolution of the luminosity function of the two samples independently as a function of redshift.  But the qualitative conclusions remain the same, and based on the data you'd have to be pretty brave to conclude very much more than that.  I'll post a few more thoughts in the comments, but those will probably only be relevant to those who have actually taken a look at this paper.