The Environment of Galaxies at Low Redshift

TL;DR

This study compares galaxy environmental effects in SDSS observations and semi-analytic models, finding qualitative agreement but also notable discrepancies in galaxy color dependence on environment.

Contribution

It evaluates the accuracy of semi-analytic models in reproducing observed environmental effects on galaxy properties at low redshift.

Findings

Semi-analytic models qualitatively match observed environmental trends.

Models overpredict blue galaxies in the field and red galaxies in clusters.

Presence of highly clustered faint red galaxies in models, potentially overlooked in surveys.

Abstract

We compare environmental effects in two analogous samples of galaxies, one from the Sloan Digital Sky Survey (SDSS) and the other from a semi-analytic model (SAM) based on the Millennium Simulation (MS), to test to what extent current SAMs of galaxy formation are reproducing environmental effects. We estimate the large-scale environment of each galaxy using a Bayesian density estimator based on distances to all ten nearest neighbors and compare broad-band photometric properties of the two samples as a function of environment. The feedbacks implemented in the semi-analytic model produce a qualitatively correct galaxy population with similar environmental dependence as that seen in SDSS galaxies. In detail, however, the colors of MS galaxies exhibit an exaggerated dependence on environment: the field contains too many blue galaxies while clusters contain too many red galaxies, compared to the SDSS sample. We also find that the MS contains a population of highly clustered, relatively faint red galaxies with velocity dispersions comparable to their Hubble flow. Such high-density galaxies, if they exist, would be overlooked in any low-redshift survey since their membership to a cluster cannot be determined due to the "Fingers of God" effect.