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Abstract: We compare environmental effects in two analogous samples of galaxies, onefrom the Sloan Digital Sky Survey SDSS and the other from a semi-analyticmodel SAM based on the Millennium Simulation MS, to test to what extentcurrent SAMs of galaxy formation are reproducing environmental effects. Weestimate the large-scale environment of each galaxy using a Bayesian densityestimator based on distances to all ten nearest neighbors and comparebroad-band photometric properties of the two samples as a function ofenvironment. The feedbacks implemented in the semi-analytic model produce aqualitatively correct galaxy population with similar environmental dependenceas that seen in SDSS galaxies. In detail, however, the colors of MS galaxiesexhibit an exaggerated dependence on environment: the field contains too manyblue galaxies while clusters contain too many red galaxies, compared to theSDSS sample. We also find that the MS contains a population of highlyclustered, relatively faint red galaxies with velocity dispersions comparableto their Hubble flow. Such high-density galaxies, if they exist, would beoverlooked in any low-redshift survey since their membership to a clustercannot be determined due to the -Fingers of God- effect.

Autor: Nicolas B. Cowan, Zeljko Ivezic


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