Physical properties, baryon content, and evolution of the Lyα forest:: New insights from high-resolution observations at z≲0.4

We present a study of the Ly alpha forest at z less than or similar to 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 Ly alpha absorbers. The cool photoionized low-z intergalactic medium (IGM) probed by narrow Ly alpha absorbers contains about 30% of the baryons. We further find that the ratio of broad to narrow Ly alpha absorbers is higher at z less than or similar to 0.4 than at 1: 5 less than or similar to z less than or similar to 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 log N-HI greater than or similar to 13.2, with a column density distribution f (N-H i) alpha N-Hi(-B) . For narrow (b <= 40 km s(-1)) absorbers, beta=1.76 +/- 0.06. The distribution of the Doppler parameter b at low redshift implies two populations: narrow (b <= 40 km s(-1)) and broad (b > 40 km s(-1)) Ly alpha absorbers (referred to as NLAs and BLAs, respectively). Both the NLAs and some BLAs probe the cool (T similar to 10(4) K) photoionized IGM. The BLAs also probe the highly ionized gas of the warm-hot IGM (T similar or equal to 10(5)-10(6) K). The distribution of b has a more prominent high-velocity tail at z less than or similar to 0.4 than at 1.5 less than or similar to z less than or similar to 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 less than or similar to 0.4 is a factor of similar to 3 higher than at 1: 5 less than or similar to z less than or similar to 3.6.