Day 379

Friday.  4 people showed up for JC yesterday.  Saturday.

1302.6594
The distribution of alpha elements in ultra-faint dwarf galaxies
Vargas, Geha, Kirby, Simon

UFDs contain some of the oldest, most metal-poor stars in the Universe.  Present metal ratios for 61 individual RGB stars across 8 UFDs.  The individual UFDs show on average lower alpha/Fe at higher metallicities, consistent with enrichment from SNIa.  Thus even the faintest galaxies have undergone at least a limited level of chemical self-enrighment.  Suggests that SF in the UFDs was not a single burst, but instead lasted at least as much as the minimum time delay of the onset of SNIa (~100 Myr) and less than ~2 Gyr.  Further show that the combined population of UFDs has an alpha/Fe abundance pattern that is inconsistent with a flat, galactic halo-like alpha abundance trend, and is also qualitatively different from that of the more luminous CVn I dSph, which does show a hit of a plateau at very low [Fe/H].

1302.6620
Galaxies in X-ray groups.  III.  Satellite color and morphology transformations
George, Ma, Bundy, Leauthaud, Tinker, Wechsler, Finoguenov, Vulcani

SFR and morphology correlate with local environment, but its process not well understood.  Galaxy groups are thought to play an important role in shaping the physical properties of galaxies before entering massive clusters at low redshift, and transformations of satellite galaxies likely dominate the buildup of local environmental correlations.  To illuminate the physical processes that shape galaxy evolution in dense environments, study a sample of 116 X-ray selected galaxy groups at z=0.2-1 with halo masses of 1e13-14 Msun and centroid determined with WL.   Analyze morphologies based on HST imaging and colors determined from 31 photometric bands for a stellar mass-limited population of 923 satellite galaxies and a comparison sample of 16k field galaxies.  Controlling for variations in stellar mass across environments, find significant trends in the colors and morphologies of satellite galaxies with group-centric distance and across cosmic time.  Specifically at low stellar mass, the fraction of SF galaxies with no prominent bulge declines from >50% among field galaxies to <20% among satellite near the centers of groups.  This decline is accompanied by a rise in dominance of quenched galaxies with intermediate bulge+disk morphologies, and only a weak increase in red bulge-dominated systems.  These results show that both color and morphology are influenced by a galaxy's location within a group halo.  Suggest that strangulation and disk fading alone are insufficient to explain the observed morphological dependence on environment, and that galaxy mergers or close tidal encounters must play a role in building up the population of quenched galaxies with bulges seen in dense environments at low redshift.  

1302.6994
Power spectrum super-sample covariance
Takada, Hu

For a wide range of survey volumes, the sample variance that arises from modes that are larger than the survey dominates the covariance of power spectrum estimates for modes much smaller than the survey.  The perturbative and deeply nonlinear versions of this effect are known as beat coupling and halo sample variance respectively.  Show that they are unified by the matter trispectrum of squeezed configurations and that such configurations obey a consistence relation which relates them to the response of the power spectrum to a change in the background density.  This method also applies to statistics that are based on radial projection of the density field such as weak lensing shear.  While they use the halo model for an analytic description to expose the nature of the effect, the consistency description enables an accurate calibration of the full effect directly from simulations.  It also suggests that super-sample covariance may be viewed as an additional interesting signal rather than excess noise.