Kavli Affiliate: Michael A. McDonald
| First 5 Authors: Josh Borrow, Mark Vogelsberger, Stephanie O’Neil, Michael A. McDonald, Aaron Smith
| Summary:
Backsplash galaxies are galaxies that once resided inside a cluster, and have
migrated back oustide as they move towards the apocentre of their orbit. The
kinematic properties of these galaxies are well understood, thanks to the
significant study of backsplashers in dark matter-only simulations, but their
intrinsic properties are not well constrained due to modelling uncertainties in
sub-grid physics, ram pressure stripping, dynamical friction, and tidal forces.
In this paper, we use the IllustrisTNG300-1 simulation, with a baryonic
resolution of $M_{rm b} approx 1.1times 10^7$ M$_odot$, to study backsplash
galaxies around 1302 isolated galaxy clusters with mass $10^{13.0} < M_{rm
200,mean} / {rm M}_odot< 10^{15.5}$. We employ a decision tree classifier to
extract features of galaxies that make them likely to be backsplash galaxies,
compared to nearby field galaxies, and find that backsplash galaxies have low
gas fractions, high mass-to-light ratios, large stellar sizes, and low black
hole occupation fractions. We investigate in detail the origins of these large
sizes, and hypothesise their origins are linked to the tidal environments in
the cluster. We show that the black hole recentreing scheme employed in many
cosmological simulations leads to the loss of black holes from galaxies
accreted into clusters, and suggest improvements to these models. Generally, we
find that backsplash galaxies are a useful population to test and understand
numerical galaxy formation models due to their challenging environments and
evolutionary pathways that interact with poorly constrained physics.
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