Seyfert's Sextet: where is the gas?

TL;DR

Deep high-resolution X-ray observations of Seyfert's Sextet reveal that most X-ray emission is from individual galaxies, with very little hot intergalactic gas, suggesting a low-mass, evolved galaxy group.

Contribution

This study provides the first high-resolution X-ray analysis of Seyfert's Sextet, clarifying the distribution of hot gas and its relation to galaxy evolution.

Findings

Most X-ray emission is from individual galaxies.

Diffuse intergalactic gas is minimal (~2x10^39 erg/s).

The group likely has a small total mass and is in an advanced evolutionary stage.

Abstract

Seyfert's Sextet (a.k.a HCG 79) is one of the most compact and isolated galaxy groups in the local Universe. It shows a prominent diffuse light component that accounts for ~50% of the total observed light. This likely indicates that the group is in an advanced evolutionary phase, which would predict a significant hot gaseous component. Previous X-ray observations had suggested a low luminosity for this system, but with large uncertainties and poor resolution. We present the results from a deep (70 ks), high resolution Chandra observation of Seyfert's Sextet, requested with the aim of separating the X-ray emission associated with the individual galaxies from that of a more extended inter-galactic component. We discuss the spatial and spectral characteristics of this group we derive with those of a few similar systems also studied in the X-ray band. The high resolution X-ray image indicates that the majority of the detected emission does not arise in the compact group but is concentrated towards the NW and corresponds to what appears to be a background galaxy cluster. The emission from the group alone has a total luminosity of ~1x10^40 erg/s in the (0.5-5) keV band. Most of the luminosity can be attributed to the individual sources in the galaxies, and only ~2x10^39 erg/s is due to a gaseous component. However, we find that this component is also mostly associated with the individual galaxies of the Sextet, leaving little or no residual in a truly IGM component. The extremely low luminosity of the diffuse emission in Seyfert's Sextet might be related to its small total mass.