Thorsten Tepper-Garcia, Philipp Richter, Joop Schaye
At z<1 a large fraction of the baryons is thought to reside in diffuse gas that has been heated to temperatures log T=5-6 by gravitational accretion shocks and galactic winds. Absorption by the 770.41A, 780.32A doublet of NeVIII in the spectra of QSOs provides us with a unique tool to study this elusive gas phase. We construct an analytic model for the NeVIII absorption as a function of the gas density, temperature, and the abundance of metals, allowing for the latter to be in-homogeneously distributed within the gaseous structures responsible for the absorption, and assuming that these structures are self-gravitating, that the gas is exposed to the UV/X-ray background, and that it is in ionisation equilibrium. Our model agrees with the predictions of a cosmological, hydrodynamic simulation from the OverWhelmingly Large Simulations project provided that the degree of metal mixing, parametrized by the average line-of-sight metal-filling fraction within the gas clouds harbouring NeVIII, is low: c_L~0.1. Most of the NeVIII at low z is produced by collisional ionisation in gas with -614, are only produced in collisionally ionised gas with T=5x10^5 K and log n_H>-4. Metal-rich ([Ne/H]=0), strong (log N~14) absorbers have typical sizes ~10 kpc and likely originate in the immediate vicinity of galaxies. Despite their relatively high total hydrogen column densities (log N_H~20), these strong NeVIII cloudlets account for less than 1% of the cosmic baryon budget. The baryon content of the gas harbouring the NeVIII absorbers is, however, a factor 1/c_L higher. We conclude that strong NeVIII absorbers are robust probes of shock-heated diffuse gas. Spectra with S/N~100 would allow the detection of the weaker systems that trace a substantial fraction of the baryons at low redshift.
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http://arxiv.org/abs/1303.6281
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