Large Scale Structure in Dark Matter and Galaxies

Abstract

Galaxy evolution and AGN growth in the early universe are believed to be strongly driven by merging (hierarchical growth) and galaxy dynamical interactions. Thus, a fall exploration of the environmental influences is absolutely essential to understanding this early evolution. The Cosmic Evolution Survey (COSMOS, [Scoville et al. 2007a]) is specifically designed to probe the correlated coevolution of galaxies, star formation, active galactic nuclei (AGN) and dark matter (DM) largescale structures (LSS) over the redshift range z > 0.5 to 6. The survey includes midti-wavelength imaging and spectroscopy from X-ray to radio wavelengths covering a 2 square degree equatorial field. Photometric redshifts are derived using 34 photometric UV-IR bands for 800,000 galaxies with accuracy reaching σ_z/(l +z) = 0.7 — 1.6% for bright galaxies (I_(AB) = 22 —24mag). Large scale structures have been traced in COSMOS from z = 0.2 to 2.5 in the baryons (from the galaxy density distribution) and in the dark matter to z = 1.1 (from weak tensing analysis of HST ACS images). These LSS extend over 20 Mpc with total mass up to ~10^(15)_ ⊙. The overall distribution of galaxy overdensities is similar with those predicted from the Millennium simulation. A trend for an increasing high overdensities at low z is clearly apparent in these data. At higher redshifts of z ~ 1, there appears to be a significant discrepancy between the observations and the simulations - with the simidations exhibiting earUer development of high density structores than is seen in the observed galaxy distributions. The observed galaxy spectral energy distributions (SEDs) and star formation rates (SFRs) clearly depend systematically on both redshift and environmental density - early SED types and lower SFRs in denser regions and at lower redshift. This evolution is probably driven by the exhaustion of the ISM and by galaxy interactions, the latter being strongly correlated with regions of highest dark matter density. Strong evolution is seen the frequency of close pairs of galaxies - particidarly for lower mass companions at projected separations 10 - 20 kpc.

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