The large-scale shock in the cluster of galaxies Hydra A

¹ Max-Planck-Institute for Extraterrestrial Physics, Giessenbachstr, 85748, Garching, Germany e-mail: aurora@mpe.mpg.de
² Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
⁴ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, the Netherlands
⁵ University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD, 21250, USA

Received: 1 October 2008
Accepted: 19 December 2008

Abstract

We analyzed a deep XMM-Newton observation of the cluster of galaxies Hydra A, focusing on the large-scale shock discovered in Chandra images as a discontinuity in the surface brightness. The shock front can be seen both in the pressure map and in temperature profiles in several sectors. We compared the results of a spherically symmetric hydrodynamic model to surface brightness profiles and temperature jumps across the shock to determine the shock properties. The Mach numbers determined from the temperature jumps are in good agreement with the Mach numbers derived from EPIC/pn surface brightness profiles and previously from Chandra data and are consistent with M ~ 1.3. In this simple model, the estimated shock age in the different sectors ranges between 130 and 230 Myr and the outburst energy between 1.5 and 3 10⁶¹ erg. The shape of the shock seen in the pressure map can be approximated with an ellipse centered ~70 kpc towards the NE from the cluster center. This is a good simple approximation to the shock shape seen in the Chandra image, although this shape shows additional small deviations from ellipticity. We aimed to develop a better model that can explain the offset between the shock center and the AGN, as well as give a consistent result on the shock age and energy. To this end, we performed 3D hydrodynamical simulations in which the shock is produced by a symmetrical pair of AGN jets launched in a spherical galaxy cluster. As an explanation of the observed offset between the shock center and the AGN, we consider large-scale bulk flows in the intracluster medium, which were included in the simulation. The simulation successfully reproduces the size, ellipticity, and average Mach number of the observed shock front. The predicted age of the shock is 160 Myr and the total input energy 3 10⁶¹ erg. Both values are within the range determined by the spherically symmetric model. To match the observed 70 kpc offset of the shock ellipse from the cluster center by large-scale coherent motions, these would need to have a high velocity of 670 km s^-1. We discuss the feasibility of this scenario and offer alternative ways to produce the observed offset and to further improve the simulation.