Constraints on the merging timescale of luminous red galaxies, or, where do all the halos go?

TL;DR

This paper investigates the merging timescales of luminous red galaxies (LRGs) within dark matter halos, finding that rapid merging or disruption is necessary to reconcile hierarchical halo growth with observed LRG counts and intracluster light.

Contribution

It provides evidence that LRGs merge or disrupt within ~2 Gyr, supporting hierarchical growth models and explaining the observed intracluster light around satellite galaxies.

Findings

LRGs must merge or disrupt within ~2 Gyr in massive halos

The stellar mass from disrupted LRGs accounts for observed intracluster light

Hierarchical halo growth is consistent with non-evolving clustering and halo occupation distribution

Abstract

In the LCDM cosmology dark matter halos grow primarily through the accretion of smaller halos. Much of the mass in a halo of 10^14Msun comes in through accretion of ~10^13Msun halos. If each such halo hosted one luminous red galaxy (LRG) then the accretion of so many halos is at odds with the observed number of LRGs in clusters unless these accreted LRGs merge or disrupt on relatively short timescales (~2Gyr). These timescales are consistent with classical dynamical friction arguments, and imply that 2-3 LRGs have merged or disrupted within each halo more massive than 10^14Msun by z=0. The total amount of stellar mass brought into these massive halos by z=0 is consistent with observations once the intracluster light (ICL) is included. If disrupted LRGs build up the ICL, then the hierarchical growth of massive halos implies that a substantial amount of ICL should also surround satellite LRGs, as suggested by recent observations of the Virgo cluster. Finally, we point out that these results are entirely consistent with a non-evolving clustering strength and halo occupation distribution, and note that observations of the latter in fact support the hypothesis that merging/disruption of massive galaxies does indeed take place at late times.