Friday, 4 December 2009

The growth of massive galaxies since z=2

From van Dokkum et al. (

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)

Friday, 27 November 2009

Restframe UV extinction laws at z~1

In this paper (, 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, 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).

Friday, 20 November 2009

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.

Friday, 6 November 2009

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.

Friday, 9 October 2009

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.

Friday, 21 August 2009

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.

Friday, 24 July 2009

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...

Friday, 10 July 2009

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.

The diversity of type 1a supernovae from broken symmetries

Stellar mass growth over cosmic time

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.

Friday, 19 June 2009

Velocity dispersion at z~2

from van Dokkum, Kriek, & Franx 2009,

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.

Friday, 12 June 2009

A new type of stellar explosion

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.

Monday, 8 June 2009

First direct metallicity at z>1

from Yuan & Kewley,

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

from Goulding & Alexander,

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.

Friday, 5 June 2009

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.

Friday, 8 May 2009

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

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

Friday, 24 April 2009

The statistical nature of the brightest cluster galaxies

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.


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.

Friday, 17 April 2009

Early Assembly of the Most Massive Galaxies

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).

Thursday, 9 April 2009


The non-ironic DVD cover:

The science:

Figure 9 of Pascale et al.,

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.

Thursday, 2 April 2009

Black hole M-sigma relation

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. (

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.

Friday, 27 March 2009

High z Low Z Galaxies

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

In, 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).

Thursday, 19 March 2009

An SMG with A_V>~5

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.

Friday, 13 March 2009

Inside-out galaxy growth?

Figure 1 from Bezanson et al.,

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).

Friday, 6 March 2009

Evolution of Lyman alpha halos around HzRG

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

from Cenarro & Trujillo,

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

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

From Dijkstra and Loeb (

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.

Friday, 20 February 2009

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).

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

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

Figure 12 from Stark et al.,

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.

Friday, 13 February 2009

Clustering of DRGs

Jeremy Tinker, Risa Wechsler and Zheng Zheng

We're right and Ryan is wrong.

A downturn in intergalactic CIV as redshift 6 is approached

From Ryan-Weber et al. ( 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).

Friday, 30 January 2009

mass build-up of red sequence

Fig 10. Ruhland et al.

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

E pur si muove: Springel,

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

From Busha et al. (

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.

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.

Friday, 23 January 2009

Excess AGN activity in clusters

from Gilmour, Best, & Almaini,

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

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

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.

Friday, 16 January 2009

Missing satellites? No problem!

Koposov et al.,

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

from Stott et al.,

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