The abundance and physical properties of O VII and O VIII X-ray
  absorption systems in the EAGLE simulations
  We use the EAGLE cosmological, hydrodynamical simulations to predict the
column density and equivalent width distributions of intergalactic O VII
($E=574$ eV) and O VIII ($E=654$ eV) absorbers at low redshift. These two ions
are predicted to account for 40% of the gas-phase oxygen, which implies that
they are key tracers of cosmic metals. We find that their column density
distributions evolve little at observable column densities from redshift 1 to
0, and that they are sensitive to AGN feedback, which strongly reduces the
number of strong (column density $N \gtrsim 10^{16} \, \mathrm{cm}^{-2})$
absorbers. The distributions have a break at $N \sim 10^{16} \,
\mathrm{cm}^{-2}$, corresponding to overdensities of $\sim 10^{2}$, likely
caused by the transition from sheet/filament to halo gas. Absorption systems
with $N \gtrsim 10^{16} \mathrm{cm}^{-2}$ are dominated by collisionally
ionized O VII and O VIII, while the ionization state of oxygen at lower column
densities is also influenced by photoionization. At these high column
densities, O VII and O VIII arising in the same structures probe systematically
different gas temperatures, meaning their line ratio does not translate into a
simple estimate of temperature. While O VII and O VIII column densities and
covering fractions correlate poorly with the H I column density at
$N_{\mathrm{H \, I}} \gtrsim 10^{15} \, \mathrm{cm}^{-2}$, O VII and O VIII
column densities are higher in this regime than at the more common, lower H I
column densities. The column densities of O VI and especially Ne VIII, which
have strong absorption lines in the UV, are good predictors of the strengths of
O VII and O VIII absorption and can hence aid in the detection of the X-ray
lines.
