AN UNUSUAL DISCONTINUITY IN THE X-RAY SURFACE BRIGHTNESS PROFILE OF NGC 507: EVIDENCE OF AN ABUNDANCE GRADIENT?

摘要

We present results from a 45 ks Chandra ACIS-Ⅰobservation of the nearby (z = 0.01646) NGC 507 group. The X-ray surface brightness profile of the outer region of the group is well described by an isothermal β-profile consistent with earlier ROSAT observations. We find a sharp edge or discontinuity in the radial surface brightness profile 55 kpc east and southeast of NGC 507 covering an ～125° arc. At the sharpest part of the discontinuity, the surface brightness declines by a factor of ～2 over a distance of 6.9 kpc (～15"). The inner and outer gas temperatures across the discontinuity differ by only about 0.2 keV (interior and exterior gas temperatures are 1.2 and 1.4 keV, respectively). Spectral analysis indicates that there is a gradient in the elemental abundance across the discontinuity, and comparison with the low-resolution NRAO VLA Sky Survey radio maps suggests that the discontinuity is aligned with a low surface brightness radio lobe. We conclude that the appearance of this discontinuity may be the result of two phenomena: the transonic (M～0.75 - 1) expansion/inflation of the radio lobe close to our line of sight and the transport of high-abundance material from the center of the galaxy due to this expansion.