Published in

Oxford University Press (OUP), Monthly Notices of the Royal Astronomical Society, 2(488), p. 2947-2969, 2019

DOI: 10.1093/mnras/stz1762

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The abundance and physical properties of O vii and O viii X-ray absorption systems in the EAGLE simulations

This paper was not found in any repository, but could be made available legally by the author.
This paper was not found in any repository, but could be made available legally by the author.

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Data provided by SHERPA/RoMEO

Abstract

Abstract We use the EAGLE cosmological, hydrodynamical simulations to predict the column density and equivalent width distributions of intergalactic O vii ($E=574 \, \rm {eV}$) and O viii ($E=654 \, \rm {eV}$) absorbers at low redshift. These two ions are predicted to account for $40 \, \hbox{ per cent}$ 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 active galactic nucleus feedback, which strongly reduces the number of strong (column density $N \gtrsim 10^{16} \, \rm {cm}^{-2}$) absorbers. The distributions have a break at $N ∼ 10^{16}\, \rm {cm}^{-2}$, corresponding to overdensities of ∼102, likely caused by the transition from sheet/filament to halo gas. Absorption systems with $N \gtrsim 10^{16} \, \rm {cm}^{-2}$ 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 ${N}_{\rm {H}\, \rm {I}} \gtrsim 10^{15} \, \rm {cm}^{-2}$, 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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