The origin of X-ray coronae around simulated disc galaxies

TL;DR

This study uses cosmological simulations to explore the origin and properties of hot X-ray emitting gas around galaxies, revealing the roles of supernovae, accretion, and feedback in shaping the hot gas halo.

Contribution

It provides a detailed analysis of the origin of X-ray coronae in simulated galaxies, highlighting the mass-dependent contributions of supernovae and accretion, and explains the low X-ray luminosity in low-mass, star-forming galaxies.

Findings

Supernova-heated gas dominates central X-ray emission in halos ≤10^{13} M_sun.

Accreted, quasi-hydrostatic gas dominates in halos ≥10^{12} M_sun.

Gas fraction and X-ray luminosity relation vary with halo mass due to AGN feedback.

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, $\leq 0.10 R_{\mathrm{vir}}$, of halos of mass $\leq 10^{13} \mathrm{M}_\odot$ 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 $\geq 10^{12} \mathrm{M}_\odot$. 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, $\leq 10^{14} \mathrm{M}_\odot$. 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.