Chandra observations of NGC4342, an optically faint, X-ray gas-rich early-type galaxy

摘要

Chandra X-ray observations of NGC4342, a low stellar mass (M_K=-22.79 mag)early-type galaxy, show luminous, diffuse X-ray emission originating from hotgas with temperature of kT~0.6 keV. The observed 0.5-2 keV band luminosity ofthe diffuse X-ray emission within the D_25 ellipse is L_0.5-2keV = 2.7 x 10^39erg/s. The hot gas has a significantly broader distribution than the stellarlight, and shows strong hydrodynamic disturbances with a sharp surfacebrightness edge to the northeast and a trailing tail. We identify the edge as acold front and conclude that the distorted morphology of the hot gas isproduced by ram pressure as NGC4342 moves through external gas. From thethermal pressure ratios inside and outside the cold front, we estimate thevelocity of NGC4342 and find that it moves supersonically (M~2.6) towards thenortheast. Outside the optical extent of the galaxy we detect ~17 bright(L_0.5-8keV > 3 x 10^37 erg/s) excess X-ray point sources. The excess sourcesare presumably low-mass X-ray binaries (LMXBs) located in metal-poor globularclusters (GCs) in the extended dark matter halo of NGC4342. Based on the numberof excess sources and the average frequency of bright LMXBs in GCs, we estimatethat NGC4342 may host roughly 850-1700 GCs. In good agreement with this,optical observations hint that NGC4342 may harbor 1200 +/- 500 GCs. This numbercorresponds to a GC specific frequency of S_N = 19.9 +/- 8.3, which is amongthe largest values observed in full-size galaxies.