In considering the connection between accretion and the formation of galaxy halos, perhaps nowhere is the process more dramatically illustrated than in the assembly of the most massive halos – the extended BCG envelopes and diffuse intracluster light (ICL) that is found in the centers of massive galaxy clusters. Unlike quiescent field galaxies whose major accretion era lies largely in the past, under hierarchical accretion scenarios, clusters of galaxies are the most recent objects to form (e.g. Fakhouri 2010); their massive central galaxies continue to undergo active assembly and halo growth even at the current epoch, and may have accreted as much as half their mass since a redshift of z = 0.5 (e.g. de Lucia & Blaizot 2007). Thus the cluster environment presents an ideal locale for studying the accretion-driven growth of massive galaxy halos.
As galaxy clusters assemble, their constituent galaxies interact with one another, first within infalling groups, then inside the cluster environment itself. Over the course of time, a variety of dynamical processes liberate stars from their host galaxies, forming and feeding the growing population of intracluster stars. This complex accretion history is illustrated in Figure 1, using the collisionless simulations of Rudick et al. (2011). At early times, individual galaxies are strewn along a collapsing filament of the cosmic web. Gravity quickly draws these galaxies into small groups, which then fall together to form larger groups. In the group environment, slow interactions between galaxies lead to strong tidal stripping and the formation of discrete tidal tails and streams. As the groups fall into the cluster, this material is efficiently mixed into the cluster ICL (Rudick et al. 2006, 2009). Concurrently, mergers of galaxies in the cluster core expel more stars into intracluster space (Murante et al. 2007), as does ongoing stripping of infalling galaxies due to interactions both with other cluster galaxies and with the cluster potential itself (Conroy et al. 2007, Purcell et al. 2007, Contini et al. 2014). Additionally, even in-situ star formation in the intracluster medium, from gas stripped from infalling galaxies, may contribute some fraction of the ICL as well (Puchwein 2010). All these processes lead to a continual growth of the intracluster light over time, as clusters continue to be fed by infalling groups and major cluster mergers. This evolution predicts that ICL properties should be linked to the dynamical state of the cluster – early in their formation history, clusters should be marked by a low total ICL fraction but with a high proportion of light in cold (and more easily visible tidal streams), while more evolved clusters would have higher ICL fractions found largely in a smooth, diffuse, and well-mixed state.
Figure 1. Formation of intracluster light during the assembly of a 1015 M⊙ galaxy cluster. The panels run from z = 2 (upper left) to the present day (lower right). From Rudick et al. (2011).
The fact that these various processes all operate concurrently makes it difficult to isolate their individual contributions to the ICL, and computational studies differ on whether group accretion, major mergers, or tidal stripping dominate the ICL. Fortunately, these processes imprint a variety of observable signatures in the ICL. The morphology and color of the diffuse light as well as the spatial distribution and kinematics of discrete ICL tracers (red giant branch (RGB) stars, planetary nebulae (PNe), and globular clusters (GCs)) all have potential to disentangle the ICL formation channels. For example, the galaxy mass-metallicity relationship predicts that stripping of low mass satellites would deposit preferentially metal-poor stars into the ICL, while mergers of massive galaxies would lead to more metal-rich ICL. Similarly the age distribution of ICL populations may differentiate between stripping of old stellar systems versus that from star-forming galaxies, or even contributions from in-situ ICL production. Thus, observational studies of the morphology, colors, kinematics, and stellar populations in the ICL are well-motivated to track the detailed accretion histories of massive clusters.