The relation between galaxies and the warm-hot circumgalactic medium probed with X-ray and UV line absorption in the EAGLE simulation

We use the EAGLE cosmological simulation to study the distribution of baryons, and FUV (O VI), EUV (Ne VIII) and X-ray (O VII, O VIII, Ne IX, and Fe XVII) line absorbers, around low-redshift galaxies and haloes of different masses. We work with column densities, but quantify their relation with equivalent widths by analysing virtual spectra. Halo gas dominates the highest column density absorption for X-ray lines, but lower-density gas contributes to strong UV absorption lines from O VI and Ne VIII. Of the O VII (O VIII) absorbers detectable in an Athena X-IFU blind survey, we find that 41 (56) per cent arise from haloes with $\mathrm{M}_{\mathrm{200c}} = 10^{12.0 - 13.5} \, \mathrm{M}_{\odot}$. We predict that the Athena X-IFU will be able to observe O VII (O VIII) covering fractions within 100 pkpc of 77 (46) per cent around $\mathrm{M}_{\star} = 10^{10.5 - 11.0} \, \mathrm{M}_{\odot}$ galaxies (59 (82) per cent for $\mathrm{M}_{\star} >10^{11.0} \, \mathrm{M}_{\odot}$), comparable to the covering fractions observable for O VI and Ne VIII by current UV spectrographs. The circumgalactic medium (CGM) contains more metals than the interstellar medium across halo masses. The ions we study here trace the warm-hot, volume-filling phase of the CGM, but are biased towards gas at temperatures corresponding to the collisional ionization peak for each ion, and towards high-metallicity gas. Gas well within the virial radius is mostly collisionally ionized, but around and beyond this radius, and for O VI, photo-ionization becomes significant. Virial-temperature collisional ionization equilibrium ion fractions are a good predictor of column density trends with halo mass, but haloes contain a larger diversity of ions than this single-temperature model predicts.