Fresh Activity in Old Systems: Radio AGN in Fossil Groups of Galaxies

TL;DR

This study systematically surveys fossil galaxy groups at 1.4 GHz, revealing prevalent radio-loud AGN activity in their central ellipticals, which may influence the intragroup medium and is linked to galaxy mass and assembly history.

Contribution

First systematic radio survey of fossil groups showing high AGN activity and its potential impact on group evolution and intragroup medium heating.

Findings

67% of fossil group candidates host radio-loud AGN

Weak correlation between AGN radio luminosity and X-ray halo luminosity

Fossil group ellipticals are significantly more luminous than typical group ellipticals

Abstract

We present the first systematic 1.4 GHz Very Large Array radio continuum survey of fossil galaxy group candidates. These are virialized systems believed to have assembled over a gigayear in the past through the merging of galaxy group members into a single, isolated, massive elliptical galaxy and featuring an extended hot X-ray halo. We use new photometric and spectroscopic data from SDSS Data Release 7 to determine that three of the candidates are clearly not fossil groups. Of the remaining 30 candidates, 67% contain a radio-loud (L_1.4GHz > 10^23 W Hz^-1) active galactic nucleus (AGN) at the center of their dominant elliptical galaxy. We find a weak correlation between the radio luminosity of the AGN and the X-ray luminosity of the halo suggesting that the AGN contributes to energy deposition into the intragroup medium. We only find a correlation between the radio and optical luminosity of the central elliptical galaxy when we include X-ray selected, elliptically dominated non-fossil groups, indicating a weak relationship between AGN strength and the mass assembly history of the groups. The dominant elliptical galaxy of fossil groups is on average roughly an order of magnitude more luminous than normal group elliptical galaxies in optical, X-ray, and radio luminosities and our findings are consistent with previous results that the radio-loud fraction in elliptical galaxies is linked to the stellar mass of a population. The current level of activity in fossil groups suggests that AGN fueling continues long after the last major merger. We discuss several possibilities for fueling the AGN at the present epoch.