Evolution of Galaxies and their Environments at z = 0.1 to 3 in COSMOS

Large-scale structures (LSS) out to z $< 3.0$ are measured in the Cosmic Evolution Survey (COSMOS) using extremely accurate photometric redshifts (photoz). The Ks-band selected sample (from Ultra-Vista) is comprised of 155,954 galaxies. Two techniques — adaptive smoothing and Voronoi tessellation — are used to estimate the environmental densities within 127 redshift slices. Approximately 250 statistically significant overdense structures are identified out to z $= 3.0$ with shapes varying from elongated filamentary structures to more circularly symmetric concentrations. We also compare the densities derived for COSMOS with those based on semi-analytic predictions for a $\Lambda$CDM simulation and find excellent overall agreement between the mean densities as a function of redshift and the range of densities. The galaxy properties (stellar mass, spectral energy distributions (SEDs) and star formation rates (SFRs)) are strongly correlated with environmental density and redshift, particularly at z $< 1.0 – 1.2$. Classifying the spectral type of each galaxy using the rest-frame b-i color (from the photoz SED fitting), we find a strong correlation of early type galaxies (E-Sa) with high density environments, while the degree of environmental segregation varies systematically with redshift out to z $\sim 1.3$. In the highest density regions, 80% of the galaxies are early types at z=0.2 compared to only 20% at z = 1.5. The SFRs and the star formation timescales exhibit clear environmental correlations. At z $> 0.8$, the star formation rate density (SFRD) is uniformly distributed over all environmental density percentiles, while at lower redshifts the dominant contribution is shifted to galaxies in lower density environments.

Comments: mp4 versions of some figures available at this http URL
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)
Cite as: arXiv:1303.6689 [astro-ph.CO]
(or arXiv:1303.6689v1 [astro-ph.CO] for this version)

Dust-Obscured Galaxies in the Local Universe

Ho Seong Hwang, Margaret J. Geller (Smithsonian Astrophysical Observatory)
(Submitted on 26 Mar 2013)

We use Wide-field Infrared Survey Explorer (WISE), AKARI, and Galaxy Evolution Explorer (GALEX) data to select local analogs of high-redshift (z~2) dust obscured galaxies (DOGs). We identify 47 local DOGs with S_{12\mu m}/S_{0.22 \mu m}>892 and S_{12\mu m}>20 mJy at 0.05<z<0.08 in the Sloan Digital Sky Survey data release 7. The infrared luminosities of these DOGs are in the range 3.4×10^{10} (L_\odot)<L_{IR}<7.0×10^{11} (L_\odot) with a median L_{IR} of 2.1×10^{11} (L_\odot). We compare the physical properties of local DOGs with a control sample of galaxies that have lower $S_{12\mu m}/S_{0.22 \mu m}$ but have similar redshift, IR luminosity, and stellar mass distributions. Both WISE 12 micron and GALEX near-ultraviolet (NUV) flux densities of DOGs differ from the control sample of galaxies, but the difference is much larger in the NUV. Among the 47 DOGs, 36\pm7% have small axis ratios in the optical (i.e., b/a<0.6), larger than the fraction among the control sample (17\pm3%). There is no obvious sign of interaction for many local DOGs. No local DOGs have companions with comparable optical magnitudes closer than ~50 kpc. The large- and small-scale environments of DOGs are similar to the control sample. Many physical properties of local DOGs are similar to those of high-z DOGs, even though the IR luminosities of local objects are an order of magnitude lower than for the high-z objects: the presence of two classes (active galactic nuclei- and star formation-dominated) of DOGs, abnormal faintness in the UV rather than extreme brightness in the mid-infrared, and diverse optical morphology. These results suggest a common underlying physical origin of local and high-z DOGs. Both seem to represent the high-end tail of the dust obscuration distribution resulting from various physical mechanisms rather than a unique phase of galaxy evolution.