Physical Properties, Baryon Content, and Evolution of the Lyα Forest: New Insights from High-Resolution Observations at z <~ 0.4

Abstract

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^-1) absorbers, β=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) 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.

Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is operated for NASA by The Johns Hopkins University under NASA contract NAS5-32985. Based on observations made with the NASA ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract No. NAS5-26555.