The unusual morphology of the intragroup medium in NGC 5171

We present the results of a 24-ks XMM-Newton observation of the NGC 5171 group of galaxies. NGC 5171 is unusual in that it is an X-ray bright group (L_X > 10⁴² erg s^-1), with irregular contours which are not centred on a bright galaxy. The global spectrum is adequately described by a single-temperature APEC model with T_X= 0.96 +/- 0.04 keV and Z= 0.13 +/- 0.02Z_solar, in good agreement with previous ROSAT data. We find that the X-ray contours are centred on a bright ridge of emission stretching from the brightest group galaxy to a nearby galaxy. Spectral mapping reveals this ridge to be both cool (T_X~ 1.1 keV) and metallic (Z~ 0.4 Z_solar) in comparison to its surroundings, suggesting it is the result of a tidal interaction between the two galaxies. Optical data reveal the member galaxies to have a high velocity dispersion (σ_v= 494 +/- 99 km s^-1) and a significantly non-Gaussian velocity distribution, suggesting that the group is in the process of merging. A region of hot gas with T_X= 1.58 +/- 0.36 keV is found to the west of the bright central ridge, and we interpret this as shock-heating resulting from the merging. A further region of emission to the south-east of the bright central ridge, with T_X= 1.14 +/- 0.13 keV, is probably associated with a background group, four times more distant.