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

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 α absorbers (BLAs) with large Doppler parameters ( 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  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 . From our simulated spectra we derive a number of BLAs per unit redshift of for H i absorbers with log (cm(km s,  km s^-1, and total hydrogen column densities N(H ii cm^-2. The baryon content of these systems is , which represents ∼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 . About 27 percent of the BLAs in our simulation originate in the photoionized Ly α forest (log ) 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.