Evolution of splashback boundaries and gaseous outskirts: insights from mergers of self-similar galaxy clusters

Abstract

A self-similar spherical collapse model predicts a dark matter (DM) splashback and accretion shock in the outskirts of galaxy clusters while misses a key ingredient of structure formation – processes associated with mergers. To fill this gap, we perform simulations of merging self-similar clusters and investigate their DM and gas evolution in an idealized cosmological context. Our simulations show that the cluster rapidly contracts during the major merger and the splashback radius rsp decreases, approaching the virial radius rvir. While in the self-similar model rsp depends on a smooth mass accretion rate parameter Γs, our simulations show that in the presence of mergers, rsp responds to the changes in the total mass accretion rate Γvir, which accounts for both mergers and smooth accretion. The scatter of the Γvir − rsp/rvir relation indicates a generally low Γs ∼ 1 in clusters in cosmological simulations. In contrast to the DM, the hot gaseous atmospheres significantly expand by the merger-accelerated (MA-) shocks formed when the runaway merger shocks overtake the outer accretion shock. After a major merger, the MA-shock radius is larger than rsp by a factor of up to ∼1.7 for Γs ≲ 1 and is ∼rsp for Γs ≳ 3. This implies that (1) mergers could easily generate the MA-shock-splashback offset measured in cosmological simulations, and (2) the smooth mass accretion rate is small in regions away from filaments where MA-shocks reside. We further discuss the shapes of the DM haloes, various shocks and contact discontinuities formed at different epochs of the merger, and the ram-pressure stripping in cluster outskirts.

Shocks in the Stacked Sunyaev-Zel'dovich Profiles of Clusters II: Measurements from SPT-SZ + Planck Compton-y Map

“…This is a common process during structure formation, and so most shocks observed in the cluster outskirts (𝑅 1𝑅 200m ) are expected to be MA-shocks. While the origin of the MA-shock is rooted in merger events, the radial evolution of the feature -once it has been generateddepends only on smooth mass accretion (Zhang et al 2021). Finally, we stress that all of these processes detailed above are complex, highly aspherical, and vary significantly from cluster to cluster.…”

“…Assuming different gas adiabatic indices in the range 𝛾 ∈ [4/3, 3], and also allowing the mass accretion rate to vary, can lead to nearly factor of 2 differences in the location of the accretion shock in self-similar solutions (Shi 2016). However, more complex, 3D hydrodynamical simulations have all found that the accretion shock feature is con-sistently located well beyond the splashback radius (Paper I; Lau et al 2015;Aung et al 2020;Zhang et al 2020Zhang et al , 2021, and this is related to the MA-shocks we discussed earlier (Zhang et al 2020).…”

“…𝑅 sh, acc /𝑅 sp 2.5 (Paper I; Aung et al 2020;Zhang et al 2021). If we assume the true location of the accretion shock in the subsample is roughly the same as its location in the fiducial sample (see Section 4.1.2 or Table 4), then the ratio is closer to 𝑅 sh, acc /𝑅 sp ≈ 4.…”

“…As was noted before, the current astrophysical picture of shocks has been studied predominantly using hydrodynamical simulations (Quilis et al 1998;Miniati et al 2000;Ryu et al 2003;Pfrommer et al 2006Pfrommer et al , 2007Kang et al 2007;Skillman et al 2008;Vazza et al 2009;Molnar et al 2009;Planelles & Quilis 2013;Vazza et al 2013Vazza et al , 2014Hong et al 2014Hong et al , 2015Lau et al 2015;Schaal & Springel 2015;Schaal et al 2016;Nelson et al 2016;Ha et al 2018;Zhang et al 2019Zhang et al , 2020Aung et al 2020;Baxter et al 2021;Zhang et al 2021;Planelles et al 2021), with the combined set of works using a wide variety of hydrodynamical solvers and astrophysical model prescriptions; see Vazza et al (2011) and Power et al (2020) for comparison studies of different implementations. These works are accompanied by observational studies of shocks that have focused predominantly on small samples -often containing just one object -of local, low-redshift clusters (Akamatsu et al 2011;Akahori & Yoshikawa 2012;Akamatsu et al 2016;Basu et al 2016;Di Mascolo et al 2019a,b;Hurier et al 2019;Pratt et al 2021;Zhu et al 2021).…”

Tempestuous life beyond R500: X-ray view on the Coma cluster with SRG/eROSITA. II. Shock & Relic

“…Yet another consequence of the merger is the possibility of accelerating some of the galaxies that were initially lagging behind the NGC4839 group to the velocities higher than the group velocity (e.g. Carucci et al 2014;Zhang et al 2021). As a consequence, these galaxies might be found well ahead of the group after the first passage through the Coma core.…”