Shock heating in the group atmosphere of the radio galaxy B2 0838+32A

TL;DR

This study presents evidence of a supersonic shock driven by a restarting radio galaxy in B2 0838+32A, marking one of the few observed cases of shock heating in the hot interstellar medium without recent merger activity.

Contribution

It provides the first direct observation of shock heating driven by accretion from hot gas in a radio galaxy without recent merger evidence.

Findings

Active lobes are expanding supersonically with Mach number ~2.4.

The host galaxy shows no signs of recent merger or cold gas reservoirs.

This is one of the few cases of shock heating observed in a young radio source.

Abstract

We present Chandra and radio observations, and analysis of Sloan Digital Sky Survey data, of the radio galaxy B2 0838+32A (4C 32.26) and its environment. The radio galaxy is at the centre of a nearby group that has often been identified with the cluster Abell 695, but we argue that the original Abell cluster is likely to be an unrelated and considerably more distant system. The radio source is a restarting radio galaxy and, using our Chandra data, we argue that the currently active lobes are expanding supersonically, driving a shock with Mach number $2.4^{+1.0}_{-0.5}$ into the inter-stellar medium. This would be only the third strong shock round a young radio source to be discovered, after Centaurus A and NGC 3801. However, in contrast to both these systems, the host galaxy of B2 0838+32A shows no evidence for a recent merger, while the AGN spectrum shows no evidence for the dusty torus that would imply a large reservoir of cold gas close to the central black hole. On the contrary, the AGN spectrum is of a type that has been associated with the presence of a radiativelyinefficient accretion flow that could be controlled by AGN heating and subsequent cooling of the hot, X-ray emitting gas. If correct, this means that B2 0838+32A is the first source in which we can directly see entropy-increasing processes (shocks) driven by accretion from the hot phase of the interstellar medium.