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NGC 4342

Title: Chandra observations of NGC4342, an optically faint, X-ray gas-rich early-type galaxy
Authors: Akos Bogdan (1), William R. Forman (1), Ralph P. Kraft (1), Christine Jones (1), Scott W. Randall (1), Zhongli Zhang (2), Irina Zhuravleva (2), Eugene Churazov (2), Zhiyuan Li (1), Paul E. J. Nulsen (1), Alexey Vikhlinin (1), Hans Boehringer (3), Sabine Schindler (4) ((1) SAO, (2) MPA, (3) MPE, (4) University of Innsbruck)

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 hot gas with temperature of kT~0.56 keV. The observed 0.5-2 keV band luminosity of the diffuse X-ray emission within the D_25 ellipse is L_0.5-2keV = 2.7 x 10^39 erg/s. The hot gas has a significantly broader distribution than the stellar light, and shows strong hydrodynamic disturbances with a sharp surface brightness edge to the northeast and a trailing tail. We identify the edge as a cold front and conclude that the distorted morphology of the hot gas is produced by ram pressure as NGC4342 moves through external gas. From the thermal pressure ratios inside and outside the cold front, we estimate the velocity of NGC4342 and find that it moves supersonically (M~2.6) towards the northeast. We also resolve eight bright (L_0.5-8keV > 3 x 10^37 erg/s) point sources within the D_25 ellipse of the galaxy, most of them being low-mass X-ray binaries (LMXBs). The luminosity of the brightest source is L_0.5-8keV = 2.6 x 10^39 erg/s and it is located in the center of NGC4342, hence we associate it with the supermassive black hole of NGC4342. Outside the optical extent of the galaxy we detect ~17 luminous excess X-ray sources. The origin of these sources is uncertain. However, a likely interpretation is that they are LMXBs located in metal-poor globular clusters in the extended dark matter halo of NGC4342. Based on the number of excess sources and the average frequency of bright LMXBs in globular clusters, we estimate that NGC4342 may host roughly 850-1700 globular clusters.

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