Simulations of thermally broadened H I Ly α 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 Ly α 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 ≈ 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 -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 Ω b (BLA) = 0.0121 h 65 -1 , 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 Ly a 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.