2021MNRAS.502.2934K

The existence of hot, accreted gaseous coronae around massive galaxies is a long-standing central prediction of galaxy formation models in the ΛCDM cosmology. While observations now confirm that extraplanar hot gas is present around late-type galaxies, the origin of the gas is uncertain with suggestions that galactic feedback could be the dominant source of energy powering the emission. We investigate the origin and X-ray properties of the hot gas that surrounds galaxies of halo mass, (10¹¹-10¹⁴) M_☉, in the cosmological hydrodynamical EAGLE simulations. We find that the central X-ray emission, <=0.10R_vir, of haloes of mass ≤10¹³ M_☉ originates from gas heated by supernovae (SNe). However, beyond this region, a quasi-hydrostatic, accreted atmosphere dominates the X-ray emission in haloes of mass ≥10¹² M_☉. We predict that a dependence on halo mass of the hot gas to dark matter mass fraction can significantly change the slope of the L_X-M_vir relation (which is typically assumed to be 4/3 for clusters) and we derive the scaling law appropriate to this case. As the gas fraction in haloes increases with halo mass, we find a steeper slope for the L_X-M_vir in lower mass haloes, ≤10¹⁴ M_☉. This varying gas fraction is driven by active galactic nuclei feedback. We also identify the physical origin of the so-called 'missing feedback' problem, the apparently low X-ray luminosities observed from high star-forming, low-mass galaxies. This is explained by the ejection of SNe-heated gas from the central regions of the halo.