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Published September 2007 | Published
Journal Article Open

Large Structures and Galaxy Evolution in COSMOS at z < 1.1


We present the first identification of large-scale structures (LSSs) at z < 1.1 in the Cosmic Evolution Survey (COSMOS). The structures are identified from adaptive smoothing of galaxy counts in the pseudo-3D space (α, δ, z) using the COSMOS photometric redshift catalog. The technique is tested on a simulation including galaxies distributed in model clusters and a field galaxy population—recovering structures on all scales from 1' to 20' without a priori assumptions for the structure size or density profile. The COSMOS photometric redshift catalog yields a sample of 1.5 × 10^5 galaxies with redshift accuracy, Δ_z_(FWHM)/(1 + z) ≤ 0.1 at z < 1.1 down to I_(AB) ≤ 25 mag. Using this sample of galaxies, we identify 42 LSSs and clusters. Projected surface-density maps for the structures indicate multiple peaks and internal structure in many of the most massive LSSs. The stellar masses (determined from the galactic SEDs) for the LSSs range from M_* ~ 10^(11) up to ~3 × 10^(13) M_☉. Five LSSs have total stellar masses exceeding 10^13 M_☉. (Total masses including nonstellar baryons and dark matter are expected to be ~50-100 times greater.) The derived mass function for the LSSs is consistent (within the expected Poisson and cosmic variances) with those derived from optical and X-ray studies at lower redshift. To characterize structure evolution and for comparison with simulations, we compute a new statistic: the area filling factor as a function of the overdensity value compared to the mean at surface overdensity (f_A [Σ/Σ(overbar)(z)). The observationally determined f_A has less than 1% of the surface area (in each redshift slice) with overdensities exceeding 10:1, and evolution to higher overdensities is seen at later epochs (lower z); both characteristics are in good agreement with what we find using similar processing on the Millennium Simulation. Although similar variations in the filling factors as a function of overdensity and redshift are seen in the observations and simulations, we do find that the observed distributions reach higher overdensities than the simulation, perhaps indicating overmerging in the simulation. All of the LSSs show a dramatic preference for earlier SED type galaxies in the denser regions of the structures, independent of redshift. The SED types in the central 1 and 1-5 Mpc regions of each structure average about one SED type earlier than the mean type at the same redshift, corresponding to a stellar population age difference of ~2-4 Gyr at z = 0.3-1. We also investigate the evolution of key galactic properties—mass, luminosity, SED, and star formation rate (SFR)—with redshift and environmental density as derived from overdensities in the full pseudo-3D cube. Both the maturity of the stellar populations and the "downsizing" of star formation in galaxies vary strongly with redshift (epoch) and environment. For a very broad mass range (10^(10)-10^(12) M_☉), we find that galaxies in dense environments tend to be older; this is not just restricted to the most massive galaxies. And in low-density environments, the most massive galaxies appear to have also been formed very early (z > 2), compared to the lower mass galaxies there. Over the range z < 1.1, we do not see evolution in the mass of galaxies by more than a factor of ~2 separating active and inactive star-forming galaxy populations.

Additional Information

© 2007 American Astronomical Society. Print publication: Issue 1 (2007 September); received 2006 April 25; accepted for publication 2006 September 22. The HST COSMOS Treasury program was supported through NASA grant HST-GO-09822. We gratefully acknowledge the contributions of the entire COSMOS collaboration consisting of more than 70 scientists. More information on the COSMOS survey is available at http://cosmos.astro.caltech.edu. The COSMOS Science meeting in 2005 May in Kyoto, Japan was supported in part by the NSF through grant OISE-0456439. Major work on this project was done while N. Z. S. was on sabbatical at the Institute for Astronomy at the University of Hawaii and during a visit at the Aspen Center For Physics. We would also like to thank the referee for a number of suggestions which have greatly improved this paper. Facilities: HST (ACS), HST (NICMOS), HST (WFPC2), Subaru (SCAM).

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