The Nature of the Low-redshift Ly-alpha Clouds

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I discuss recent HST observations and interpretation of low-z Ly-alpha clouds toward nearby Seyferts and QSOs, including their frequency, space density, estimated mass, association with galaxies, and contribution to Omegab. Our HST/GHRS detections of 73 Lya absorbers with NₕI > 10¹2.6 cm(-2) along 11 sightlines covering pathlength Delta(cz) = 86,000 km/s show f(>NₕI) proportional to NₕI(-0.6 +/- 0.2). A group of strong absorbers toward PKS 2155-304 may be associated with gas (400-800)/h₇5 kpc from 4 large galaxies with low metallicity (<0.004 solar) and D/H < 2 x 10-4. At low-z, we derive a metagalactic ionizing radiation field and Lya-forest baryon density J₀ = (1.1 +/- 0.4) x 10(-23) ergs/cm²/s/Hz/sr and Omegab = (0.008 +/- 0.004) h₇5(-1) [J-23 N₁4 b₁00]⁰.5 for clouds of characteristic size b = (100 kpc)b₁00.