Resonant Scattering Effect on the Soft X-Ray Line Emission from the Hot Interstellar Medium. I. Galactic Bulges

Diffuse soft X-ray line emission is commonly used to trace the thermal and chemical properties of the hot interstellar medium, as well as its content, in nearby galaxies. Although resonant-line scattering complicates the interpretation of the emission, it also offers an opportunity to measure the kinematics of the medium. We have implemented a direct Monte Carlo simulation scheme that enables us to account for the resonant scattering (RS) effect in the medium, in principle, with arbitrary spatial, thermal, chemical, and kinematic distributions. Here we apply this scheme via dimensionless calculation to an isothermal, chemically uniform, and spherically symmetric medium with a radial density distribution characterized by a β-model. This application simultaneously accounts for both optical depth-dependent spatial distortion and intensity change of the resonant-line emission due to the scattering, consistent with previous calculations. We further apply the modeling scheme to the O VII and O VIII emission line complex observed in the XMM-Newton RGS spectrum of the M31 bulge. This modeling, although with various limitations due to its simplicity, shows that the RS could indeed account for much of the spatial distortion of the emission, as well as the relative intensities of the lines, especially the large forbidden-to-resonant-line ratio of the O VII Heα triplet. We estimate the isotropic turbulence Mach number of the medium in M31 as ∼0.17 for the first time and the line-emitting gas temperature as ∼2.3 × 10⁶ K. We conclude that the RS may in general play an important role in shaping the soft X-ray spectra of diffuse hot gas in normal galaxies.