Hubble and FUSE studies of Ly-alpha absorbers at low redshift

Abstract

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(solar). A comparison of Ly-beta/Ly-alpha ratios (FUSE and HST) yields a distribution of Doppler parameters with mean 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 Ly-alpha profile fitting, with mean b(CoG)/b(width) = 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.