Thorsten Tepper-García, Philipp Richter, Joop Schaye, Craig M. Booth, Claudio Dalla Vecchia, Tom Theuns
We investigate the physical state of HI absorbing gas at low redshift
(z=0.25) using a subset of cosmological, hydrodynamic simulations from the OWLS
project, focusing in particular on broad (b_HI > 40 km/s) Lyman-Alpha absorbers
(BLAs), which are believed to originate in shock-heated gas in the Warm-Hot
Intergalactic Medium (WHIM). Our fiducial model, which includes radiative
cooling by heavy elements and feedback by supernovae and active galactic
nuclei, predicts that by z=0.25 nearly 60 per cent of the gas mass ends up at
densities and temperatures characteristic of the WHIM and we find that half of
this fraction is due to outflows. The standard HI observables (distribution of
HI column densities N_HI, distribution of Doppler parameters b_HI, b_HI - N_HI
correlation) and the BLA line number density predicted by our simulations are
in remarkably good agreement with observations.
BLAs arise in gas that is hotter, more highly ionised and more enriched than
the gas giving rise to typical Lyman-Alpha forest absorbers. Although the
majority of the BLAs arise in warm-hot (log(T/K) ~ 5) gas at low (log Delta <
1.5) densities, their line width correlates only weakly with the gas
temperature, and is thus a poor indicator of the thermal state of the gas.
Detectable BLAs account for only a small fraction of the true baryon content of
the WHIM at low redshift. In order to detect the bulk of the mass in this gas
phase, a sensitivity at least one order of magnitude better than achieved by
current UV spectrographs is required. We argue that BLAs mostly trace gas that
has been shock-heated and enriched by outflows and that they therefore provide
an important window on a poorly understood feedback process.
View original:
http://arxiv.org/abs/1201.5641
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