Markarian 6 : shocking the environment of an intermediate Seyfert

Abstract

Markarian 6 is a nearby (D ~ 78 Mpc) Seyfert 1.5, early-type galaxy, with a double set of radio bubbles. The outer set spans similar to 7.5 kpc and is expanding into the halo regions of the host galaxy. We present an analysis of our new Chandra observation, together with archival XMM-Newton data, to look for evidence of emission from shocked gas around the external radio bubbles, both from spatially resolved regions in Chandra and from spectral analysis of the XMM-Newton data. We also look for evidence of a variable absorbing column along our line of sight to Mrk 6, to explain the evident differences seen in the active galactic nucleus (AGN) spectra from the various, non-contemporaneous, observations. We find that the variable absorption hypothesis explains the differences between the Chandra and XMM-Newton spectra, with the Chandra spectrum being heavily absorbed. The intrinsic N-H varies from similar to 8 x 10(21) atoms cm(-2) to similar to 3 x 10(23) atoms cm(-2) on short timescales (2-6 years). The past evolution of the source suggests this is probably caused by a clump of gas close to the central AGN, passing in front of us at the moment of the observation. Shells of thermal X-ray emission are detected around the radio bubbles, with a temperature of similar to 0.9 keV. We estimate a temperature of similar to 0.2 keV for the external medium using luminosity constraints from our Chandra image. We analyze these results using the Rankine-Hugoniot shock jump conditions, and obtain a Mach number of similar to 3.9, compatible with a scenario in which the gas in the shells is inducing a strong shock in the surrounding interstellar medium (ISM). This could be the third clear detection of strong shocks produced by a radio-powerful Seyfert galaxy. These results are compatible with previous findings on Centaurus A and NGC 3801, supporting a picture in which these AGN-driven outflows play an important role in the environment and evolution of the host galaxy.