X-ray signatures of black hole feedback: hot galactic atmospheres in IllustrisTNG and X-ray observations

Hot gaseous atmospheres that permeate galaxies and extend far beyond their stellar distribution, where they are commonly referred to as the circumgalactic medium, imprint important information about feedback processes powered by the stellar populations of galaxies and their central supermassive black holes (SMBHs). In this work, we study the properties of this hot X-ray emitting medium using the IllustrisTNG cosmological simulations. We analyse their mock X-ray spectra, obtained from the diffuse and metal-enriched gas in TNG100 and TNG50, and compare the results with X-ray observations of nearby early-type galaxies. The simulations reproduce the observed X-ray luminosities (L_X) and temperature (T_X) at small (<R_e) and intermediate (<5R_e) radii reasonably well. We find that the X-ray properties of lower mass galaxies depend on their star formation rates. In particular, in the magnitude range where the star-forming and quenched populations overlap, $M_{\rm K}\sim -24\ (M_*\sim 10^{10.7}\, \mathrm{M}_\odot)$ , we find that the X-ray luminosities of star-forming galaxies are on average about an order of magnitude higher than those of their quenched counterparts. We show that this diversity in L_X is a direct manifestation of the quenching mechanism in the simulations, where the galaxies are quenched due to gas expulsion driven by SMBH kinetic feedback. The observed dichotomy in L_X is thus an important observable prediction for the SMBH feedback-based quenching mechanisms implemented in state-of-the-art cosmological simulations. While the current X-ray observations of star-forming galaxies are broadly consistent with the predictions of the simulations, the observed samples are small and more decisive tests are expected from the sensitive all-sky X-ray survey with eROSITA.