The nature of the ghost cavity in the NGC 741 group

TL;DR

This paper investigates a large X-ray emitting bubble in the NGC 741 galaxy group, analyzing its energy source, composition, and the implications for galaxy group dynamics and past nuclear activity.

Contribution

It provides new constraints on the electron energy distribution and magnetic field in the bubble, challenging the assumption that it is a relic of typical radio source evolution.

Findings

The bubble's energy output can offset radiative cooling in the intra-group medium.

Constraints suggest a very low high-energy cutoff in the electron spectrum.

The bubble's properties do not match those expected from a typical dead radio source.

Abstract

We discuss the effects of energy injection into the intra-group medium of the group of galaxies associated with NGC 741. The X-ray emission reveals a large bubble, which in the absence of a currently bright central radio source, may have been inflated by a previous cycle of nuclear activity . If the bubble is filled with a light, relativistic fluid we calculate that in expanding, it has done more than sufficient work to counteract the energy lost from the intra-group medium via radiative cooling; the bubble can provide this energy as it expands and rises. Using upper limits on the flux density of the plasma filling the bubble at 330 MHz and 1.4 GHz, we derive constraints on its electron energy distribution and magnetic field strength. We show that the data require the high-energy cut-off of the electron spectrum to be very low compared to the cut-offs seen in more typical radio sources if the fluid filling the bubble is a conventional relativistic plasma. This suggests that the fluid filling the bubble may not have evolved by expansion or synchrotron losses consistent with a dead radio source, leaving a puzzle as to what the origin of the bubble may be.