Far Ultraviolet Spectroscopic Explorer observations of the low-redshift Lyβ forest

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 Ly beta absorbers that correspond to Ly alpha 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 m Angstrom (4 sigma) Ly beta detection limits, we have employed the equivalent width ratio of Ly beta/Ly alpha, and occasionally higher Lyman lines, to determine the Doppler parameter b and accurate column densities N-H, for moderately saturated lines. We detect Ly beta absorption corresponding to all Ly alpha lines with W-lambda greater than or equal to 200 m Angstrom. The Ly beta/Ly alpha 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-H1 approximate to 50,000 K, although the inferred Doppler parameters are considerably less than the widths derived from Ly alpha profile fitting, (b/b(width)) = 0.52. The typical increase in column density over that derived from profile fitting is Delta log N-H1 = 0.3, but ranges up to 1.0 dex. Our data suggest that the low-z Lya, absorbers contain sizable nonthermal motions or velocity components in the line profile, perhaps arising from cosmological expansion and infall.