An X-Ray Absorption-line Survey of the Circumgalactic Medium of M31 with XMM-Newton

Abstract X-ray absorption-line spectroscopy provides a powerful method for probing the extended hot circumgalactic medium (CGM) of individual galaxies. In this study, we present the first survey of O vii and O viii absorption lines toward the hot halo of M31, using 15 and 18 background active galactic nuclei, with impact parameters ranging from R imp ∼ 300–730 kpc and ∼190–730 kpc, respectively. We find a marginal excess absorption above the expected Milky Way (MW) foreground toward the innermost sightlines, consistent with an additional hot-CGM contribution associated with M31. Comparing sightlines inside and outside R imp = 300–350 kpc, the excess corresponds to a mean equivalent width of ∼4–9 mÅ for the inner O vii sightlines at R imp ∼ 310 kpc and ∼17 mÅ for the inner O viii sightlines at R imp ∼ 200 kpc, with a nominal combined significance level of 2.2 σ –2.3 σ . The hot-CGM mass inferred from this excess is highly model dependent, especially on the assumed CGM boundary, and therefore does not yet provide a robust baryon census. Attributing the excess entirely to hot gas confined within approximately the virial radius of M31 would require a CGM mass exceeding the nominal “missing” baryon budget. The inferred mass decreases to a few ×10 11 M ⊙ when the assumed CGM boundary is extended to 400–500 kpc. Deeper observations, both through longer exposures of existing sightlines and the inclusion of additional inner-halo targets, will be essential to robustly constrain the properties of the hot CGM of M31.