Chandra Detection of Diffuse Hot Gas in and around the M31 Bulge

Abstract

We report the detection of diffuse hot gas in M31, using archivalChandra observations that allow us to map out a 30^{'}{\times}30^{'}field (covering a galactocentric radius up to 4.5 kpc) and to detectsources in the galaxy down to a 0.5-8 keV luminosity limit of ~10^{35}ergs s^{-1}. We estimate the remaining stellar contribution fromfainter X-ray sources (primarily cataclysmic variables and coronallyactive binaries), assuming that they spatially follow the stellardistribution. Indeed, the near-IR K-band light of the galaxy closelytraces the 2-8 keV unresolved X-rays, indicating a collective stellarX-ray emissivity consistent with those determined for the Galacticridge and M32, whereas the amount of the 0.5-2 keV unresolved emissionis significantly greater than the expected stellar contribution,especially within a galactocentric radius of ~2 kpc. Morphologically,this soft X-ray excess appears substantially rounder than the bulgeas seen in K band and is elongated approximately along the minor-axisat large radii. The excess thus most likely represents the emissionof diffuse hot gas in and around the galactic bulge. Furthermore,the near side of the M31 disk casts an apparent shadow against thesoft X-ray excess, indicating that the hot gas extends to at least2.5 kpc from the galactic plane. We briefly discuss the implicationsof these results on the energy balance in the M31 bulge and on understandingthe large-scale soft X-ray enhancement observed toward the innerregion of our own Galaxy.