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 $\Lambda$CDM 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, $(10^{11}-10^{14}){\mathrm{M}}_{\odot }$, in the cosmological hydrodynamical EAGLE simulations. We find that the central X-ray emission, $\le 0.10R_{\mathrm{vir}}$, of halos of mass $\le 10^{13}{\mathrm{M}}_{\odot }$ 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 $\ge 10^{12}{\mathrm{M}}_{\odot }$. We predict that a dependence on halo mass of the hot gas to dark matter mass fraction can significantly change the slope of the $L_{\mathrm{X}}-M_{\mathrm{vir}}$ 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 halos increases with halo mass, we find a steeper slope for the $L_{\mathrm{X}}-M_{\mathrm{vir}}$ in lower mass halos, $\le 10^{14}{\mathrm{M}}_{\odot }$. 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.

18 pages, 11 figures, accepted for publication in MNRAS

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