Predictions for the X-ray circumgalactic medium of edge-on discs and spheroids

Abstract

ABSTRACT We investigate how the X-ray circumgalactic medium (CGM) of present-day galaxies depends on galaxy morphology and azimuthal angle using mock observations generated from the EAGLE cosmological hydrodynamic simulation. By creating mock stacks of eROSITA-observed galaxies oriented to be edge-on, we make several observationally testable predictions for galaxies in the stellar mass range M⋆ = 1010.7–11.2 M⊙. The soft X-ray CGM of disc galaxies is between 60 and 100 per cent brighter along the semimajor axis compared to the semiminor axis, between 10 and 30 kpc. This azimuthal dependence is a consequence of the hot (T > 106 K) CGM being non-spherical: specifically, it is flattened along the minor axis such that denser and more luminous gas resides in the disc plane and corotates with the galaxy. Outflows enrich and heat the CGM preferentially perpendicular to the disc, but we do not find an observationally detectable signature along the semiminor axis. Spheroidal galaxies have hotter CGMs than disc galaxies related to spheroids residing at higher halo masses, which may be measurable through hardness ratios spanning the 0.2–1.5 keV band. While spheroids appear to have brighter CGMs than discs for the selected fixed M⋆ bin, this owes to spheroids having higher stellar and halo masses within that M⋆ bin, and obscures the fact that both simulated populations have similar total CGM luminosities at the exact same M⋆. Discs have brighter emission inside 20 kpc and more steeply declining profiles with radius than spheroids. We predict that the eROSITA 4-yr all-sky survey should detect many of the signatures we predict here, although targeted follow-up observations of highly inclined nearby discs after the survey may be necessary to observe some of our azimuthally dependant predictions.