Galaxy evolution in merging clusters: The passive core of the “Train Wreck” cluster of galaxies, A 520

Abstract

Aims. The mergers of galaxy clusters are the most energetic events in the universe after the Big Bang. With the increased availability of multi-object spectroscopy and X-ray data, an ever increasing fraction of local clusters are recognised as exhibiting signs of recent or past merging events on various scales. Our goal is to probe how these mergers affect the evolution and content of their member galaxies. We specifically aim to answer the following questions: Is the quenching of star formation in merging clusters enhanced when compared with relaxed clusters? Is the quenching preceded by a (short-lived) burst of star formation? Methods. We obtained optical spectroscopy of > 400 galaxies in the field of the merging cluster Abell 520. We combine these observations with archival data to obtain a comprehensive picture of the state of star formation in the members of this merging cluster. Finally, we compare these observations with a control sample of ten non-merging clusters at the same redshift from The Arizona Cluster Redshift Survey (ACReS). We split the member galaxies into passive, star forming or recently quenched depending on their spectra. Results. The core of the merger shows a decreased fraction of star forming galaxies compared to clusters in the non-merging sample. This region, dominated by passive galaxies, is extended along the axis of the merger. We find evidence of rapid quenching of the galaxies during the core passage with no signs of a star burst on the time scales of the merger ( 0.4 Gyr). Additionally, we report the tentative discovery of an infalling group along the main filament feeding the merger, currently at ∼2.5 Mpc from the merger centre. This group contains a high fraction of star forming galaxies as well as approximately two thirds of all the recently quenched galaxies in our survey.

A Complete Spectroscopic Census of A2029: A Tale of Three Histories

“…N AGN /N member , is ∼ 3%. The AGN fraction is similar to the AGN fractions measured in individual clusters (∼ 0.5 − 9%, Deshev et al 2017;Habas et al 2018). However, a direct comparison among the AGN fractions is not trivial because the magnitude limit and spectroscopic completeness of other cluster redshift surveys vary.…”

Inside a Beehive: The Multiple Merging Processes in the Galaxy Cluster Abell 2142*

“…This hypothesis was denoted the "remnant core" scenario (Markevitch et al 2000), and is appropriate for merging clusters with prominent signatures of recent or ongoing mergers. However, A2142 shows an almost regular morphology and appears relaxed at large radii, unlike 1E 0657-56 (Markevitch et al 2002), A520 (Govoni et al 2001;Markevitch et al 2005;Deshev et al 2017), and other clusters with cold fronts, which clearly appear unrelaxed. Therefore, Tittley & Henriksen (2005); Markevitch & Vikhlinin (2007); Owers et al (2011) proposed an alternative model, where the observed cold fronts derive from a sloshing cool core Chu-razov et al 2003).…”

Jellyfish: Ram Pressure Stripping As a Diagnostic Tool in Studies of Cluster Collisions∗ †

“…Although the lopsided distribution and high amplitudes of radial velocities of our RPS candidates heavily disfavor the specific merger geometry adopted historically for A1758N, the observational evidence nonetheless strongly supports a causal link between RPS and merger events in clusters. Previous studies found two opposing effects of such a link: Stroe et al (2015Stroe et al ( , 2017 and Ruggiero et al (2019) report evidence of RPS triggered by merger-induced shocks, while other studies (Pranger et al 2014;Deshev et al 2017) find the fraction of starforming galaxies in mergers reduced compared to nonmerging clusters, possibly after a preceding short starburst phase. Although seemingly in conflict with each other at face value, these findings might be reconciled as part of a bigger picture in which RPS events in mergers first trigger an initial burst of star formation, and then reduce or completely quench star formation as the supply of atomic and molecular gas is either exhausted or removed from the affected galaxies.…”