Abstract
ABSTRACT In the outer regions of a galaxy cluster, galaxies either may be falling into the cluster for the first time or have already passed through the cluster centre at some point in their past. To investigate these two distinct populations, we utilize TheThreeHundred project, a suite of 324 hydrodynamical resimulations of galaxy clusters. In particular, we study the ‘backsplash population’ of galaxies: those that have passed within R200 of the cluster centre at some time in their history, but are now outside of this radius. We find that, on average, over half of all galaxies between R200 and 2R200 from their host at $z$ = 0 are backsplash galaxies, but that this fraction is dependent on the dynamical state of a cluster, as dynamically relaxed clusters have a greater backsplash fraction. We also find that this population is mostly developed at recent times ($z$ ≲ 0.4), and is dependent on the recent history of a cluster. Finally, we show that the dynamical state of a given cluster, and thus the fraction of backsplash galaxies in its outskirts, can be predicted based on observational properties of the cluster.
Highlights
The Lambda cold dark matter model of the Universe describes the hierarchical formation of cosmological structure
A common definition is one based purely on the present-day locations of galaxies; the backsplash population consists of galaxies that have passed within the virial radius of a cluster at some previous time, but are found outside of the cluster, at some distance D > Rvir from the cluster centre (Gill et al 2005; Baheet al. 2013), the ‘virial radius’ used in this definition is open to interpretation
We note that, when considering fs, part of the correlation between this parameter and the backsplash fraction is caused by the backsplash galaxies themselves; the movement of a large number of galaxies from within R200 to the cluster outskirts will reduce the amount of substructure within R200, causing the fraction of mass contained in subhaloes, fs, to decrease
Summary
The Lambda cold dark matter model of the Universe describes the hierarchical formation of cosmological structure. The presence of a ‘splashback radius’ in galaxy clusters indicates that material can leave a cluster and re-enter at a later stage Theoretical work on this radius includes that of Adhikari, Dalal & Chamberlain (2014), Diemer & Kravtsov (2014), and More, Diemer & Kravtsov (2015), who each describe the splashback radius as the distance from a cluster centre at which accreting matter first reaches the apocentre of its orbit, and show that this radius physically corresponds to the distance at which the cluster density profile drops most steeply. As no assumptions are made about the boundness of backsplash galaxies, they could travel far beyond the splashback radius, rather than remaining on bound orbits This would be analogous to the ‘renegade subhaloes’ identified by Knebe et al (2011b) in simulations of the Local Group, which were associated with a host halo, but entered a different host at a later time.
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