FAR ULTRAVIOLET SPECTROSCOPIC EXPLORER OBSERVATIONS OF THE LOW-REDSHIFT Lyβ FOREST

摘要

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 Lya 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 mA (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_(H I) for moderately saturated lines. We detect Lyβ absorption corresponding to all Lya lines with W_λ ≥ 200 mA. The Lyβ/Lyα ratios yield a preliminary distribution function of Doppler parameters, with mean < b > = 31.4 ± 7.4 km s~(-1) and median 28km s~(-1), comparable to values at redshifts z = 2.0-2.5. If thermal, these b-values correspond to T_(H I) ≈ 50,000 K, although the inferred Doppler parameters are considerably less than the widths derived from Lya profile fitting, < b/b_(width) > = 0.52. The typical increase in column density over that derived from profile fitting is Δ log N_(H I) = 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.