Encounters of merger and accretion shocks in galaxy clusters and their effects on intracluster medium

Abstract

Several types/classes of shocks naturally arise during formation and evolution of galaxy clusters. One such class is represented by accretion shocks, associated with deceleration of infalling baryons. Such shocks, characterized by a very high Mach number, are present even in 1D models of cluster evolution. Another class is composed of "runaway merger shocks", which appear when a merger shock, driven by a sufficiently massive infalling subcluster, propagates away from the main-cluster center. We argue that, when the merger shock overtakes the accretion shock, a new long-living shock is formed that propagates to large distances from the main cluster (well beyond its virial radius) affecting the cold gas around the cluster. We refer to these structures as Merger-accelerated Accretion shocks (MA-shocks) in this paper.We show examples of such MA-shocks in 1D and 3D simulations and discuss their characteristic properties. In particular, (1) MA-shocks shape the boundary separating the hot intracluster medium (ICM) from the unshocked gas, giving this boundary a "flower-like" morphology. In 3D, MA-shocks occupy space between the dense accreting filaments.(2) Evolution of MA-shocks highly depends on the Mach number of the runaway merger shock and the mass accretion rate parameter of the cluster. (3) MA-shocks may lead to the misalignment of the ICM boundary and the splashback radius.(racc rsp) if the gas adiabatic index is γ = 5/3 (see e.g. Shi 2016b) 1 .However, the evolution of galaxy clusters is more complicated than those one-dimensional (1D) self-similar solutions. Two major processes tend to break the self-similarity (and also spherical symmetry) of galaxy clusters, i.e. active galactic nucleus (AGN) feedback (see e.g. Werner et al. 2019, for a recent review) and cluster mergers (e.g. Sarazin 2002). The former process perturbs the gas in cluster cores (e.g. 100 kpc); the latter one, 1 Specifically, the alignment of the racc and rsp holds when γ = 5/3 only if the cluster mass accretion rate parameter (Γ) is in the range of 0.5 ≤ Γ ≤ 5 (Shi 2016b; see the definition of Γ in Section 2).

Impact of baryons in cosmic shear analyses with tomographic aperture mass statistics

“…This expelled material can be deposited in low-density regions, which likely explains the reduction in pixels with highly negative S/N. We note that the cause of the latter effect is not yet fully understood and could involve other baryonic processes such as interaction of internal and accretion shocks (e.g., Zhang et al 2020). The increase of the distribution at S/N close to zero accounts for the density smoothing due to this redistribution of matter which leads to a higher number of small S/N structures.…”

Gas distribution from clusters to filaments in IllustrisTNG

“…Cluster peripheries are thus crucial places to probe the transition between hot and warm gas, and to probe the complex dynamics of infalling WHIM gas. In fact, this is in these regions that gas is expected to flow from filaments into clusters, with turbulent motions (Rost et al 2021;Vallés-Pérez et al 2021), and where accretion shocks might arise (Shi et al 2020;Zhang et al 2020Zhang et al , 2021bZhu et al 2021b). Following these findings on the radial gas properties, we focus in the rest of the paper our exploration of the azimuthal gas distribution in cluster environment concentrating on two main gas phases in two different radial apertures: hot medium up to 1 R 200 and WHIM from 1 to 2R 200 .…”

Exploring the role of cosmological shock waves in the Dianoga simulations of galaxy clusters

“…Quilis et al 1998;Miniati et al 2001;Ryu et al 2003;Kang et al 2007;Skillman et al 2008;Vazza et al 2009bVazza et al , 2010Vazza et al , 2011bPlanelles and Quilis 2013;Martin-Alvarez et al 2017), particle-based (e.g. Pfrommer et al 2006;Hoeft et al 2008;Vazza et al 2011b;Zhang et al 2020) or moving-mesh simulations (Schaal and Springel 2015;Schaal et al 2016). While most of these studies are based on non-radiative simulations (e.g.…”

“…This process takes place through the heating of the gas by stronger (external) accretion shocks, with M ∼ 10 − 100, developed in the outskirts of clusters and around filaments. Given that the position of these accretion shocks results from the collision of internal and external shocks (Zhang et al 2020), both kind of shocks contribute to the total kinetic energy deposited in the medium. Numerical simulations, however, tend to produce significant differences in some quantities related to the injection of CRs by DSA mechanisms, limiting our current understanding on this process.…”