2000ApJ...538L..13S

We describe a moderate-resolution (20-25 km.s^–1) Far Ultraviolet Spectroscopic Explorer (FUSE) study of the low-redshift intergalactic medium. We report on studies of seven extragalactic sight lines and 12 Lyβ absorbers that correspond to Lyα lines detected by Hubble Space Telescope (HST)/Goddard High Resolution Spectrograph and Space Telescope Imaging Spectrograph. These absorbers appear to contain a significant fraction of the low-z baryons and were a major discovery of the HST spectrographs. Using FUSE data, with 40 mÅ (4 σ) Lyβ detection limits, we have employed the equivalent width ratio of Lyβ/Lyα, and occasionally higher Lyman lines, to determine the Doppler parameter b and accurate column densities N_HI_ for moderately saturated lines. We detect Lyβ absorption corresponding to all Lyα lines with W_λ≥200 mÅ. The Lyβ/Lyα ratios yield a preliminary distribution function of Doppler parameters, with mean <b≥31.4±7.4 km.s^–1 and median 28 km.s^–1, comparable to values at redshifts z=2.0-2.5. If thermal, these b-values correspond to T_HI_~50,000 K, although the inferred Doppler parameters are considerably less than the widths derived from Lyα profile fitting, <b/b_width≥0.52. The typical increase in column density over that derived from profile fitting is ΔlogN_HI_=0.3, but ranges up to 1.0 dex. Our data suggest that the low-z Lyα absorbers contain sizable nonthermal motions or velocity components in the line profile, perhaps arising from cosmological expansion and infall.