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 Ly \alpha , Ly \beta , 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 } ^ { -1.8 } 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 0.003 Z _ { \odot } .
A comparison of Ly \beta /Ly \alpha ratios ( FUSE and HST ) yields a distribution of Doppler parameters with \langle b \rangle = 31.4 \pm 7.4 km s ^ { -1 } and median 28 km s ^ { -1 } , comparable to values at z = 2–3 .
The curve-of-growth ( CoG ) b -values are considerably less than widths derived from Ly \alpha profile fitting , \langle b _ { CoG } / b _ { width } \rangle = 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 \Omega _ { IGM } and metallicities for the strong Ly \alpha 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 .