Accretion of galaxy groups into galaxy clusters

Abstract

We study the role of group infall in the assembly and dynamics of galaxy clusters in ΛCDM. We select 10 clusters with virial mass M200 ∼ 1014$\rm M_\odot$from the cosmological hydrodynamical simulation Illustris and follow their galaxies with stellar mass M⋆ ≥ 1.5 × 108$\rm M_\odot$. A median of $\sim 38\%$ of surviving galaxies at z = 0 are accreted as part of groups and did not infall directly from the field, albeit with significant cluster-to-cluster scatter. The evolution of these galaxy associations is quick, with observational signatures of their common origin eroding rapidly in 1-3 Gyr after infall. Substructure plays a dominant role in fostering the conditions for galaxy mergers to happen, even within the cluster environment. Integrated over time, we identify (per cluster) an average of 17 ± 9 mergers that occur in infalling galaxy associations, of which 7 ± 3 occur well within the virial radius of their cluster hosts. The number of mergers shows large dispersion from cluster to cluster, with our most massive system having 42 mergers above our mass cut-off. These mergers, which are typically gas rich for dwarfs and a combination of gas rich and gas poor for M⋆ ∼ 1011$\rm M_\odot$, may contribute significantly within ΛCDM to the formation of specific morphologies, such as lenticulars (S0) and blue compact dwarfs in groups and clusters.

Is Abell 2261 a fossil galaxy cluster in a transitional dynamical state?

“…However, these features were likely made from a much earlier major merger that fully coalesced during the early cluster formation, rather than the recent minor group passage we consider here. Also, If it is passed the center of the cluster, the group has to be broken (Benavides et al 2020).…”

Evolution of galaxy scaling relations in clusters at 0.5 <z< 1.5

“…Furthermore, mergers are more likely to occur in the outskirts of the clusters than in their cores (Deger et al 2018;Bahé et al 2019), and minor mergers are also more frequent than major mergers in galaxy groups and cluster outskirts (Benavides et al 2020), as it happens in the field (Lotz et al 2011;Kaviraj et al 2015;Ventou et al 2019). The loss of angular momentum through minor mergers in star-forming galaxies before they reach the cluster core could explain why we do not see significant differences in f j between the population of infalling and accreted cluster regions, as mergers are unlikely to occur in the cluster core, and other angular momentum redistribution mechanisms such as ram-pressure stripping may quench these galaxies rather quickly, excluding them from our samples.…”

Compact Groups of Galaxies in Sloan Digital Sky Survey and LAMOST Spectral Survey. I. The Catalogs

“…Indeed, numerical simulations (e.g., Cohn 2012;Choque-Challapa et al 2019) have predicted that these newly accreted systems have not passed through the cluster center and are mainly located at the outskirts of host clusters. Despite of experiencing dynamical heating, these sub-systems remain dynamically colder and more compact than the host cluster (Benavides et al 2020). After the first pericentric passage, they would soon be disassembled and virialized within the host cluster.…”

“…On the other hand, it is also possible that these embedded CGs are gravitationally bound sub-systems recently falling into large clusters. In this case, the dynamics of their members could be partially heated but still be kinematically colder than the host clusters (e.g., Choque-Challapa et al 2019;Benavides et al 2020)).…”