Abstract
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 -6<log nH<-4, 5<log T<6, and NeVIII column densities log
N<14. Absorbers as strong as those recently detected by COS, log N>14, 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.
Description
[1303.6281] Absorption signatures of warm-hot gas at low redshift: NeVIII
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