The Complex X‐Ray Morphology of NGC 7618: A Major Group‐Group Merger in the Local Universe?

We present results from a short {\em Chandra}/ACIS-S observation of NGC 7618,the dominant central galaxy of a nearby ($z$=0.017309, d=74.1 Mpc) group. Wedetect a sharp surface brightness discontinuity 14.4 kpc N of the nucleussubtending an angle of 130$^\circ$ with an X-ray tail extending $\sim$ 70 kpcin the opposite direction. The temperature of the gas inside and outside thediscontinuity is 0.79$\pm$0.03 and 0.81$\pm$0.07 keV, respectively. There ismarginal evidence for a discontinuous change in the elemental abundance($Z_{inner}$=0.65$\pm$0.25,$Z_{outer}$=0.17$\pm$0.21 at 90% confidence),suggesting that this may be an `abundance' front. Fitting a two-temperaturemodel to the ASCA/GIS spectrum of the NGC 7618/UGC 12491 pair shows thepresence of a second, much hotter ($T$=$\sim$2.3 keV) component. We considerseveral scenarios for the origin of the edge and the tail including a radiolobe/IGM interaction, non-hydrostatic `sloshing', equal-mass merger andcollision, and ram-pressure stripping. In the last case, we consider thepossibility that NGC 7618 is falling into UGC 12491, or that both groups arefalling into a gas poor cluster potential. There are significant problems withthe first two models, however, and we conclude that the discontinuity and tailare most likely the result of ram pressure stripping of the NGC 7618 group asit falls into a larger dark matter potential.Comment: 20 pages, 6 figures, 2 tables - accepted for publication in the Astrophysical Journa