The environmental history of group and cluster galaxies in a LambdaCDM Universe

TL;DR

This study uses cosmological simulations to explore the environmental history of galaxies in groups and clusters, revealing intrinsic biases and the importance of hierarchical growth in galaxy evolution.

Contribution

It provides new insights into the timing and processes of galaxy accretion and satellite formation within the LambdaCDM framework, highlighting the role of pre-processing and assembly history.

Findings

Massive satellites are accreted later than less massive ones.

Incomplete mixing during halo assembly links satellite timing to current position.

Long timescales (~5-7 Gyr) are needed for star formation suppression in satellites.

Abstract

We use publicly available galaxy merger trees, obtained applying semi-analytic techniques to a large high resolution cosmological simulation, to study the environmental history of group and cluster galaxies. Our results highlight the existence of an intrinsic history bias which makes the nature versus nurture (as well as the mass versus environment) debate inherently ill posed. In particular we show that: (i) surviving massive satellites were accreted later than their less massive counterparts, from more massive haloes; (ii) the mixing of galaxy populations is incomplete during halo assembly, which creates a correlation between the time a galaxy becomes satellite and its present distance from the parent halo centre. The weakest trends are found for the most massive satellites, as a result of efficient dynamical friction and late formation times of massive haloes. A large fraction of the most massive group/cluster members are accreted onto the main progenitor of the final halo as central galaxies, while about half of the galaxies with low and intermediate stellar mass are accreted as satellites. Large fractions of group and cluster galaxies (in particular those of low stellar mass) have therefore been `pre-processed' as satellites of groups with mass ~10^13 Msun. To quantify the relevance of hierarchical structure growth on the observed environmental trends, we have considered observational estimates of the passive galaxy fractions, and their variation as a function of halo mass and cluster-centric distance. Comparisons with our theoretical predictions require relatively long times (~5-7 Gyr) for the suppression of star formation in group and cluster satellites. [abridged]