Hubble and FUSE Studies of Ly α Absorbers at Low z

Ultraviolet spectrographs aboard the Hubble Space Telescope (HST) and the Far Ultraviolet Spectroscopic Explorer (FUSE) have proved their value as sensitive probes of the low-density intergalactic medium (IGM) at low redshifts (z < 0.1). Recent observations in Lyα, Lyβ, and occasional higher Lyman lines show that warm photoionized gas in the low-z IGM may contain 20–25% of the baryons, with a N HI distribution in column density. Measurements of resonance lines of Si III, C III, C IV, and O VI suggest that the metallicity of these absorbers ranges from 1– 10% of solar abundance down to values below 0.003Z⊙. A comparison of Lyβ/Lyα ratios (FUSE and HST) yields a distribution of Doppler parameters with 〈b〉 = 31.4± 7.4 km s and median 28 km s, comparable to values at z = 2–3. The curve-of-growth (CoG) b-values are considerably less than widths derived from Lyα profile fitting, 〈bCoG/bwidth〉 = 0.52, which suggests that low-z absorbers contain sizable non-thermal motions or velocity components arising from cosmological expansion and infall. A challenge for future UV spectroscopic missions (HST/COS and SUVO) is to obtain precision measurements of ΩIGM and metallicities for the strong Lyα absorbers that dominate the IGM baryon content. This program will require accurate determinations of: (1) curves of growth using higher Lyman series lines; (2) the ionizing radiation field at 1–5 Ryd; and (3) characteristic sizes and shapes of the absorbers.