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.

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.

Velocity dispersion at z~2

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

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

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

A new type of stellar explosion

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

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

First direct metallicity at z>1

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

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

AGN activity in nearby galaxies

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

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

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

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

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

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

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

The statistical nature of the brightest cluster galaxies

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.