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

Abstract

Recent far-ultraviolet (FUV) absorption line measurements of low-redshift quasars have unveiled a population of intervening broad HI Lya absorbers (BLAs) with large Doppler parameters (b> 40 km/s). 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 ~ 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=0. From our simulated spectra we derive a number of BLAs per unit redshift of (dN/dz)_BLA ~ 38 for HI absorbers with log (N(cm^-2)/b(km/s))>10.7, b>40 km/s, and log N(HII)<20.5. The baryon content of these systems is Omega_b(BLA)=0.0121/h_65, 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 T~4.4-6.2. About 27 percent of the BLAs in our simulation originate in the photoionized Lya 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.