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We report the detection of diffuse hot gas in M31, using archival Chandraobservations which allow us to map out a 30' by 30' field (covering agalactocentric radius up to 4.5 kpc) and to detect sources in the galaxy downto a 0.5-8 keV luminosity limit of ~10^35 ergs/s. We estimate the remainingstellar contribution from fainter X-ray sources (primarily cataclysmicvariables and coronally active binaries), assuming that they spatially followthe stellar distribution. Indeed, the near-IR K-band light of the galaxyclosely traces the 2-8 keV unresolved X-rays, indicating a collective stellarX-ray emissivity consistent with those determined for the Galactic ridge andM32, whereas the amount of the 0.5-2 keV unresolved emission is significantlygreater than the expected stellar contribution, especially within agalactocentric radius of ~2 kpc. Morphologically, this soft X-ray excessappears substantially rounder than the bulge as seen in K-band and is elongatedapproximately along the minor-axis at large radii. The excess thus most likelyrepresents the emission of 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 least 2.5 kpc fromthe galactic plane. We briefly discuss the implications of these results on theenergy balance in the M31 bulge and on understanding the large-scale soft X-rayenhancement observed toward the inner region of our own Galaxy.

Chandra detection of diffuse hot gas in and around the M31 bulge