Tracing baryons in the warm-hot intergalactic medium with broad Ly α absorption

Abstract

We discuss physical properties and baryonic content of broad Ly α absorbers (BLAs) at low redshift. These absorption systems, recently discovered in high-resolution, high-signal to noise quasar absorption line spectra, possibly trace the warm-hot intergalactic medium (WHIM) in the temperature range between 10⁵ and 10⁶ K. The central idea is that in ionization equilibrium WHIM filaments should contain a very small fraction of neutral gas (f_H I∼ 10^-5{-}10^-6, typically), giving rise to weak intervening H i Ly α absorption. Due to the high temperature of the WHIM, these Ly α absorbers must be thermally broadened to Doppler parameters (b values) ≥ 40 km s^-1. It is expected, however, that also non-thermal line broadening processes, line blends, and noise features can mimic broad spectral features, complicating the quantitative estimate of the baryon content of the BLAs. To extend previous BLA measurements we have reanalyzed archival STIS data of the two quasars H 1821+643 and PG 0953+415 and have identified 13 BLA candidates along a total (unblocked) redshift path of Δ z=0.440. Combining our measurements with previous results for the lines of sight toward PG 1259+593 and PG 1116+215, the resulting new BLA sample consists of 20 reliably detected systems as well as 29 additional tentative cases, implying a BLA number density of dN_BLA/dz=22{-}53. Eight BLAs show associated absorption from H i Ly β and/or O vi. The comparison between Ly α, Ly β, and O vi line widths suggests that non-thermal broadening and noise features substantially affect the observed BLA b value distribution. However, it remains unclear whether BLAs and O vi absorbers trace the same gas phase in the WHIM filaments. We estimate that the contribution of BLAs to the baryon density at z=0 is Ω_b(BLA)≥ 0.0027 h₇₀ ^-1 for absorbers with log [N(cm^-2)/b(km s^-1)] ≳ 11.3. This number indicates that WHIM broad Ly α absorbers contain a substantial fraction of the baryons in the local Universe.