The abundance and physical properties of O vii and O viii X-ray absorption systems in the EAGLE simulations. Issue 2 (27th June 2019)

• Record Type: Journal Article
• Authors: Wijers, Nastasha A; Schaye, Joop; Oppenheimer, Benjamin D; Crain, Robert A; Nicastro, Fabrizio
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 488:Issue 2(2019)
• Page Start: 2947
• Page End: 2969
• Publication Date: 2019-06-27
• Subjects: galaxies: evolution -- galaxies: haloes -- intergalactic medium -- quasars: absorption lines; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stz1762
• Languages: English
• ISSNs: 0035-8711

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 $N \sim 10^{16}\, \rm {cm}^{-2}$, corresponding to overdensities of ∼10 2, 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.