The origin of X-ray coronae around simulated disc galaxies
Abstract
The existence of hot, accreted gaseous coronae around massive galaxies is a long-standing central prediction of galaxy formation models in the 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, ≤ 0.10 Rvir, of halos 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 halos 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 LX-Mvir 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 halos increases with halo mass, we find a steeper slope for the LX-Mvir in lower mass halos, ≤ 1014 M. This varying gas fraction is driven by active galactic nuclei (AGN) 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.