Extended X-ray emission from non-thermal sources in the COSMOS field: A detailed study of a large radio galaxy at z=1.168

TL;DR

This study investigates the contribution of non-thermal X-ray emission from radio galaxies in the COSMOS field, finding only one candidate source dominated by inverse Compton processes, thus suggesting limited contamination in X-ray group samples.

Contribution

It provides a systematic analysis of non-thermal X-ray emission from radio galaxies, highlighting their rarity and impact on X-ray selected group and cluster samples.

Findings

Only one candidate with non-thermal X-ray emission identified among 60 radio galaxies.

External inverse Compton emission from lobes is the main process producing X-ray emission.

Non-thermal radio galaxy emission is unlikely to significantly contaminate X-ray group/cluster samples.

Abstract

X-ray selected galaxy group samples are usually generated by searching for extended X- ray sources that reflect the thermal radiation of the intragroup medium. On the other hand, large radio galaxies that regularly occupy galaxy groups also emit in the X-ray window, and their contribution to X-ray selected group samples is still not well understood. In order to investigate their relative importance, we have carried out a systematic search for non-thermal extended X-ray sources in the COSMOS field. Based on the morphological coincidence of X-ray and radio extensions, out of 60 radio galaxies, and \sim 300 extended X-ray sources, we find only one candidate where the observed extended X-ray emission arises from non- thermal processes related to radio galaxies. We present a detailed analysis of this source, and its environment. Our results yield that external Inverse Compton emission of the lobes is the dominant process that generates the observed X-ray emission of our extended X-ray candidate, with a minor contribution from the gas of the galaxy group hosting the radio galaxy. Finally, we show that finding only one potential candidate in the COSMOS field (in a redshift range 0 < z < 6 and with radio luminosity between 1025 and 1030 W/Hz) is consistent with expected X-ray-counts arising from synchrotron lobes. This implies that these sources are not a prominent source of contamination in samples of X-ray selected clusters/groups, but they could potentially dominate the z > 1 cluster counts at the bright end (S_X > 7 \cdot 10^-15 erg s^-1 cm^2).