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

We discuss physical properties and baryonic content of broad Ly alpha 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 10(5) and 10(6) K. The central idea is that in ionization equilibrium WHIM filaments should contain a very small fraction of neutral gas (f(HI) similar to 10(-5) - 10(-6), typically), giving rise to weak intervening Hi Ly a absorption. Due to the high temperature of the WHIM, these Ly a 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 H1821+643 and PG0953+415 and have identified 13 BLA candidates along a total (unblocked) redshift path of Delta z = 0.440. Combining our measurements with previous results for the lines of sight toward PG1259+ 593 and PG1116+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 Hi Ly beta and/ or O VI. The comparison between Ly alpha, Ly beta, and O VI line widths suggests that non- thermal broadening and noise features substantially a. ect 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 ohm b(BLA) >= 0.0027 h(70)(-1) for absorbers with log [ N(cm(-2))/b(km s(-1))] >= 11.3. This number indicates that WHIM broad Ly alpha absorbers contain a substantial fraction of the baryons in the local Universe.