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# The accretion histories of brightest cluster galaxies from their stellar population gradients

Sarah Brough, Jimmy, Kim-Vy Tran, Warrick J. Couch, Richard M. McDermid, Chris Lidman, Anja von der Linden, Rob Sharp

galaxies: clusters: general – galaxies: elliptical and lenticular, cD – galaxies: evolution – galaxies: kinematics and dynamics – galaxies: stellar content – galaxies: structure

# Introduction

\label{sec:intro} Brightest Cluster Galaxies (BCGs) are extremely luminous galaxies that are usually located in the centre of rich galaxy clusters. They have been shown to be distinct from other similarly luminous cluster galaxies (e.g. Hausman et al., 1978; Postman et al., 1995; Lazzati et al., 1998; von der Linden et al., 2007). In the hierarchical scenario of structure formation (Toomre, 1977; White et al., 1978) galaxies grow in mass and size by merging with their neighbours. BCGs are predicted to have a more active merger history than lower mass galaxies . These galaxies are often considered as the extreme end-point of massive galaxy evolution. However, despite being among the most luminous galaxies, and generally easy to detect, observations and theory have not reached a common point yet, and their evolution is still not fully understood.

Observations suggest that the mass growth of BCGs evolves with time. BCGs accrete their mass at a fast rate until $$z\sim0.5$$, thereafter their mass growth slows down (Lidman et al., 2013; Lin et al., 2013; Oliva-Altamirano et al., 2014; Inagaki et al., 2015). Studies looking at BCG companions have concluded that their stellar mass grows through major mergers ($$\ge1:3$$ mass ratios) by a factor of $$1.8\pm 0.43$$ at $$0.8 < z < 1.5$$ (Burke et al., 2013), and mostly by minor mergers ($$\leq1:4$$ mass ratios) by a factor of $$1.1$$ at $$z < 0.3$$ (Edwards et al., 2012). Major mergers are rare at low redshifts, yet still possible (e.g. Brough et al., 2011; Jimmy et al., 2013).

The recent accretion history of galaxies can be read through their stellar population gradients. In the canonical scenario, a galaxy’s initial metallicity gradient is set by an initial starburst at $$z\geq3$$ and the metallicity decreases in the outskirts, as metallicity follows the changes in the gravitational potential (Scott et al., 2009; McDermid et al., 2012). This gradient can be disrupted by violent merging events (major mergers), or reinfor