Galaxy evolution in merging clusters: The passive core of the "Train Wreck" cluster of galaxies, A520

TL;DR

This study investigates how galaxy mergers influence star formation, revealing rapid quenching in the core of the merging cluster Abell 520 without evidence of starburst activity, and identifies an infalling group rich in star-forming and recently quenched galaxies.

Contribution

It provides new observational evidence of rapid star formation quenching during cluster mergers and characterizes the galaxy population in the merging cluster Abell 520.

Findings

Decreased fraction of star-forming galaxies in the merger core.

Rapid quenching of galaxies during core passage (~0.4 Gyr).

Identification of an infalling group with high star-forming and recently quenched galaxies.

Abstract

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? We obtained optical spectroscopy of >400 galaxies in the field of the merging cluster Abell 520. We combine these observations with archival data to get 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 10 non-merging clusters at the same redshift from The Arizona Cluster Redshift Survey (ACReS). We split the member galaxies in passive, star forming or recently quenched depending on their spectra. 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 (~0.4Gyr). 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 ~2/3 of all the recently quenched galaxies in our survey.