Inferring the Mass of the Circumgalactic Medium Using X-Ray Resonant Scattering

Abstract

The circumgalactic medium (CGM) regulates galaxy growth and retains the imprint of feedback from supernovae and supermassive black holes. However, the bulk of the hot CGM produces little X-ray emission and is challenging to study with X-ray telescopes. We propose a novel method for evaluating the CGM mass using resonant scattering of the helium-like oxygen (O VII) resonant line at E = 574 eV. In a spherically symmetric and static CGM halo with a sharp central X-ray peak, the number of O VII ions within an outer radial shell can be calculated from the ratio of two directly observable quantities: the O VII flux from the bright inner region and the scattered O VII flux from the shell. To evaluate the accuracy of this geometric estimate for realistic galaxies with satellites, anisotropies, and gas velocities, we use a sample from the TNG50 cosmological simulation. When the most irregular systems are excluded based on their X-ray observables, we accurately predict the O VII mass in the outer halo, for example in an R₅₀₀c-R₂₀₀c shell, with only a 10% bias and an rms scatter of approximately 0.2 dex. As O VII mass strongly correlates with total oxygen and gas mass, this direct O VII-counting method enables indirect estimates of those quantities with future X-ray microcalorimeter missions such as NewAthena and HUBS.