An X-ray Absorption-Line Survey of the Circumgalactic Medium of M31 with XMM-Newton

X-ray absorption line spectroscopy provides a powerful method for probing the extended hot circumgalactic medium (CGM) of individual galaxies. In this study, the authors 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, respectively.

They compared sightlines inside and outside an impact parameter of R_imp = 300–350 kpc to isolate any excess beyond the expected Milky Way foreground. The impact parameters ranged from ~300–730 kpc for the O VII sample and ~190–730 kpc for the O VIII sample.

The survey found a marginal excess absorption toward the innermost sightlines, consistent with an additional hot-CGM contribution associated with M31. 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 2.2–2.3σ.

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, while extending the assumed boundary to 400–500 kpc reduces the inferred mass to a few ×10^11 M_sun. Deeper observations, 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.