The Nature of the Low-redshift Lyα Clouds
نویسنده
چکیده
I discuss recent HST observations and interpretation of low-z Lyα clouds toward nearby Seyferts and QSOs, including their frequency, space density, estimated mass, association with galaxies, and contribution to Ωb. Our HST/GHRS detections of 73 Lyα absorbers with NHI ≥ 10 12.6 cm along 11 sightlines covering pathlength ∆(cz) = 86, 000 km s show f(> NHI) ∝ N −0.6±0.2 HI . A group of strong absorbers toward PKS 2155-304 may be associated with gas (400 − 800)h 75 kpc from 4 large galaxies, with low metallicity (≤ 0.004 solar) and D/H ≤ 2 × 10. At low-z, we derive a metagalactic ionizing radiation field and Lyα-forest baryon density J0 = (1.1 ± 0.4) × 10 −23 ergs cm s Hz sr and Ωb = (0.008 ± 0.004)h 75 [J−23N14b100] 1/2 for clouds of characteristic size b = (100 kpc)b100.
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