The abundance and physical properties of O VII and O VIII X-ray absorption systems in the EAGLE simulations. (arXiv:1904.01057v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Wijers_N/0/1/0/all/0/1">Nastasha A. Wijers</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Schaye_J/0/1/0/all/0/1">Joop Schaye</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Oppenheimer_B/0/1/0/all/0/1">Benjamin D. Oppenheimer</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Crain_R/0/1/0/all/0/1">Robert A. Crain</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nicastro_F/0/1/0/all/0/1">Fabrizio Nicastro</a>
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.
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.
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