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

¹ Tartu Observatory, 61602 Tõravere, Estonia
e-mail: boris.deshev@to.ee
² Institute of Physics, University of Tartu, Ravila 14c, 50411 Tartu, Estonia
³ Department of Physics, University of Helsinki, PO Box 64, 00014 Helsinki, Finland
⁴ Max-Planck-Institute for Extraterrestrial Physics, Giessenbachstraße, 85748 Garching, Germany
⁵ Institute for Astronomy (IfA), University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
⁶ School of Physics, Korea Institute for Advanced Study, 85 Hoegiro, Dongdaemun-gu, 02455 Seoul, Republic of Korea
⁷ Quantum Universe Center, Korea Institute for Advanced Study, 85 Hoegiro, Dongdaemun-gu, 02455 Seoul, Republic of Korea
⁸ INAF–Osservatorio Astronomico di Brera, via Brera 28, 20122 Milano; via E. Bianchi 46, 23807 Merate, Italy
⁹ National Centre for Radio Astrophysics, TIFR, Ganeshkhind, 411007 Pune, India
¹⁰ Korea Astronomy and Space Science Institute, 776 Daedeokdae-Ro, Yuseong-Gu, 34055 Daejeon, Republic of Korea

Received: 24 May 2017
Accepted: 10 July 2017

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.