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

Recent far-ultraviolet (FUV) absorption line measurements of low-redshift quasars have unveiled a population of intervening broad HI Ly alpha absorbers (BLAs) with large Doppler parameters (b >= 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 approximate to 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 approximate to 0. From our simulated spectra we derive a number of BLAs per unit redshift of (dN/dz)(BLA) approximate to 38 for HI absorbers with log (N(cm(-2))/b(km s(-1))) >= 10.7, b >= 40 km s(-1), and total hydrogen column densities N(H II) = 10(20.5) cm(-2). The baryon content of these systems is Omega(b)(BLA) = 0.0121 h(65)(-1), which represents similar to 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 approximate to 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 Hi column densities and b values.