Seyfert’s Sextet: where is the gas?
1 INAF – Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
2 Instituto de Astrofisica de Andalucia, 18008 Granada, Spain
3 University of Wisconsin – Stevens Point, Department of Physics and Astronomy, 2001 Fourth Avenue, Stevens Point, WI 54481-1957, USA
4 Instituto de Astronomia, Universidad Nacional Autonoma de Mexico (UNAM), Apdo. Postal 70-264, 04510 Mexico, D.F., Mexico
Received: 29 December 2011
Accepted: 13 March 2012
Aims. 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.
Methods. 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.
Results. 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 ~1 × 1040 erg s-1 in the (0.5–5) keV band. Most of the luminosity can be attributed to the individual sources in the galaxies, and only ~2 × 1039 erg s-1 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.
Key words: galaxies: groups: general / galaxies: groups: individual: HGC 79 / intergalactic medium / X-rays: galaxies
© ESO, 2012