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 H i 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{-}10⁶ 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 H i 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 h₆₅ ^-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 α 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.