2007ApJ...658..680L

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 seven 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 logN_HI_≳13.2, with a column density distribution f(N_HI_)∝N-β^_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∼10⁴ K) photoionized IGM. The BLAs also probe the highly ionized gas of the warm-hot IGM (T≃10⁵-10⁶ 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.