Investigating the cores of fossil systems with Chandra

¹ Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: bharadwaj@astro.uni-bonn.de
² Max-Planck-Institut für Extraterrestriche Physik, Giessenbachstrasse 1, 85748 Garching, Germany

Received: 5 February 2015
Accepted: 9 September 2015

Abstract

Aims. We aim to systematically investigate the cores of a sample of fossil galaxy groups and clusters (“fossil systems”), using Chandra data, to see what hints they can offer about the properties of the intracluster medium in these particular objects.

Methods. We chose a sample of 17 fossil systems from literature with archival Chandra data and determined the cool-core fraction for fossils via three observable diagnostics, namely the central cooling time, cuspiness, and concentration parameter. We quantified the dynamical state of the fossils by the X-ray peak/brightest cluster galaxy (BCG) separation, and the X-ray peak/emission weighted centre separation. We also investigated the X-ray emission coincident with the brightest cluster galaxy (BCG) to detect the presence of potential thermal coronae. A deprojection analysis was performed for fossils with z< 0.05 to resolve subtle temperature structures, and to obtain the cooling time and entropy profiles. We also investigated the L_X − T relation for fossils from the 400d catalogue to test whether the scaling relation deviates from what is typically observed for other groups.

Results. Most fossils are identified as cool-core objects via at least two cool-core diagnostics with the population of weak cool-core fossils being the highest. All fossils have their dominant elliptical galaxy within 50 kpc of the X-ray peak, and most also have the emission weighted centre within that distance. We do not see clear indications of an X-ray corona associated with the BCG unlike coronae observed for some other clusters. Fossils lack universal temperature profiles, with some low-temperature objects generally not showing features that are expected for ostensibly relaxed objects with a cool-core. The entropy profiles of the z< 0.05 fossil systems can be described well by a power law with shallower indices than what is expected for pure gravitational processes. Finally, the fossils L_X − T relation shows indications of an elevated normalisation with respect to other groups, which seems to persist even after factoring in selection effects.

Conclusions. We interpret these results within the context of the formation and evolution of fossils, and speculate that non-gravitational heating, and AGN feedback in particular, could have had an impact on the ICM properties of these systems.