A textbook example of ram-pressure stripping in the Hydra A/A780 cluster

¹ INAF–Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
e-mail: sabrina.degrandi@brera.inaf.it
² Department of Astronomy, University of Geneva, ch. d’Ecogia 16, 1290 Versoix, Switzerland
³ INAF–IASF-Milano, via E. Bassini 15, 20133 Milano, Italy
⁴ Dipartimento di Fisica dell’Università degli Studi di Trieste, Sezione di Astronomia, via Tiepolo 11, 34143 Trieste, Italy
⁵ INAF–Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
⁶ E.A. Milne Centre for Astrophysics, Department of Physics & Mathematics, University of Hull, Hull, HU6 7RX, UK
⁷ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
⁸ Università degli Studi Milano, via Celoria 16, 20133 Milano, Italy

Received: 30 May 2015
Accepted: 18 February 2016

Abstract

In the current epoch, one of the main mechanisms driving the growth of galaxy clusters is the continuous accretion of group-scale halos. In this process, the ram pressure applied by the hot intracluster medium on the gas content of the infalling group is responsible for stripping the gas from its dark-matter halo, which gradually leads to the virialisation of the infalling gas in the potential well of the main cluster. Using deep wide-field observations of the poor cluster Hydra A/A780 with XMM-Newton and Suzaku, we report on the discovery of an infalling galaxy group 1.1 Mpc south of the cluster core. The presence of a substructure is confirmed by a dynamical study of the galaxies in this region. A wake of stripped gas is trailing behind the group over a projected scale of 760 kpc. The temperature of the gas along the wake is constant at kT ~ 1.3 keV, which is about a factor of two less than the temperature of the surrounding plasma. We observe a cold front pointing westwards compared to the peak of the group, which indicates that the group is currently not moving in the direction of the main cluster, but is moving along an almost circular orbit. The overall morphology of the group bears remarkable similarities with high-resolution numerical simulations of such structures, which greatly strengthens our understanding of the ram-pressure stripping process.