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Issue A&A
Volume 451, Number 3, June I 2006
Page(s) 767 - 776
Section Cosmology (including clusters of galaxies)
DOI http://dx.doi.org/10.1051/0004-6361:20054556



A&A 451, 767-776 (2006)
DOI: 10.1051/0004-6361:20054556

Simulations of thermally broadened H I Ly $\alpha$ absorption arising in the warm-hot intergalactic medium

P. Richter1, T. Fang2 and G. L. Bryan3

1  Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
    e-mail: prichter@astro.uni-bonn.de
2  Department of Astronomy, University of California at Berkeley, 601 Campbell Hall, Berkeley, CA 94720, USA
3  Department of Physics, University of Oxford, Keble Road, Oxford OX13RH, UK

(Received 21 November 2005 / Accepted 8 February 2006)

Abstract
Recent far-ultraviolet (FUV) absorption line measurements of low-redshift quasars have unveiled a population of intervening broad H I Ly $\alpha$ absorbers (BLAs) with large Doppler parameters ($b\geq 40$ km s-1). If the large width of these lines is dominated by thermal line broadening, the BLAs may trace highly-ionized gas in the warm-hot intergalactic medium (WHIM) in the temperature range $T\approx 10^5{-}10^6$ K, a gas phase that is expected to contain a large fraction of the baryons at low redshift. In this paper we use a hydrodynamical simulation to study frequency, distribution, physical conditions, and baryon content of the BLAs at $z\approx 0$. From our simulated spectra we derive a number of BLAs per unit redshift of $({\rm d}{\cal N}/{\rm d}z)_{\rm BLA}\approx 38$ for H I absorbers with log (N(cm-2)/b(km s $^{-1}))\geq 10.7$, $b\geq 40$ km s-1, and total hydrogen column densities N(H II $)\leq 10^{20.5}$ cm-2. The baryon content of these systems is $\Omega_{\rm b}{\rm (BLA)}=0.0121\,h_{65}\,^{-1}$, which represents $\sim $25 percent of the total baryon budget in our simulation. Our results thus support the idea that BLAs represent a significant baryon reservoir at low redshift. BLAs predominantly trace shock-heated collisionally ionized WHIM gas at temperatures log  $T\approx 4.4{-}6.2$. About 27 percent of the BLAs in our simulation originate in the photoionized Ly $\alpha$ forest (log T<4.3) and their large line widths are determined by non-thermal broadening effects such as unresolved velocity structure and macroscopic turbulence. Our simulation implies that for a large-enough sample of BLAs in FUV spectra it is possible to obtain a reasonable approximation of the baryon content of these systems solely from the measured H I column densities and b values.


Key words: methods: numerical -- cosmology: miscellaneous -- large-scale structure of Universe




© ESO 2006


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