Physical Properties, Baryon Content, and Evolution of the Lyα Forest: New Insights from High Resolution Observations

We present a study of the Lyα forest at z . 0.4 from which we conclude that at least 20% of the total baryons in the universe are located in the highly-ionized gas traced by broad Lyα absorbers. The cool photoionized low-z intergalactic medium (IGM) probed by narrow Lyα absorbers contains about 30% of the baryons. We further find that the ratio of broad to narrow Lyα absorbers is higher at z . 0.4 than at 1.5 . z . 3.6, implying that a larger fraction of the low redshift universe is hotter and/or more kinematically disturbed. We base these conclusions on an analysis of 7 QSOs observed with both FUSE and the HST/STIS E140M ultraviolet echelle spectrograph. Our sample has 341 H I absorbers with a total unblocked redshift path of 2.064. The observed absorber population is complete for logNHI & 13.2, with a column density distribution f(NHI) ∝ N HI . For narrow (b ≤ 40 km s) absorbers β = 1.76 ± 0.06. The distribution of the Doppler parameter b at low redshift implies two populations: narrow (b ≤ 40 km s) and broad (b > 40 km s) Lyα absorbers (referred to as NLAs and BLAs, respectively). Both the NLAs and some BLAs probe the cool (T ∼ 10 K) photoionized IGM. The BLAs also probe the highly-ionized gas of the warm-hot IGM (T ≃ 10–10 K). The distribution of b has a more prominent high velocity tail at z . 0.4 than at 1.5 . z . 3.6, which results in median and mean b-values that are 15–30% higher at low z than at high z. The ratio of the number density of BLAs to NLAs at z . 0.4 is a factor of ∼3 higher than at 1.5 . z . 3.6. Subject headings: cosmology: observations — intergalactic medium — quasars: absorption lines