Implications of the remarkable homogeneity of galaxy groups and clusters

TL;DR

This study finds that galaxy groups and clusters exhibit remarkably uniform star formation histories across different masses, suggesting environmental effects on galaxy evolution occur primarily below M=1E13/h Msun.

Contribution

It provides the first robust statistical constraints on the diversity of galaxy populations in groups and clusters, highlighting the homogeneity across a wide mass range.

Findings

Intrinsic scatter in passive galaxy fraction is very small (<0.1) across most mass bins.

No significant trend of diversity with increasing galaxy group mass.

Environmental effects on galaxy evolution likely occur below M=1E13 Msun.

Abstract

We measure the diversity of galaxy groups and clusters with mass M>1E13/h Msun, in terms of the star formation history of their galaxy populations, for the purpose of constraining the mass scale at which environmentally-important processes play a role in galaxy evolution. We consider three different group catalogues, selected in different ways, with photometry and spectroscopy from the Sloan Digital Sky Survey. For each system we measure the fraction of passively-evolving galaxies within R200 and brighter than either Mr=-18 (and with z<0.05) or Mr=-20 (and z<0.1). We use the (u-g) and (r-i) galaxy colours to distinguish between star-forming and passively-evolving galaxies. By considering the binomial distribution expected from the observed number of members in each cluster, we are able to either recover the intrinsic scatter in this fraction, or put robust 95% confidence upper-limits on its value. The intrinsic standard deviation in the fraction of passive galaxies is consistent with a small value of <0.1 in most mass bins for all three samples. There is no strong trend with mass; even groups with M=1E13/h Msun are consistent with such a small, intrinsic distribution. We compare these results with theoretical models of the accretion history to show that, if environment plays a role in transforming galaxies, such effects must occur first at mass scales far below that of rich clusters, at most M=1E13 Msun.