A Cluster of Low-Redshift [CLC]Ly[/CLC]α Clouds toward PKS 2155−304. I. Limits on Metals and D/H

We report observations from the Hubble Space Telescope (HST) and the VLA onthe galactic environment, metallicity, and D/H in strong low-redshift Lyaabsorption systems toward the bright BL Lac object PKS 2155-304. GHRS/G160Mspectra at 20 km/s resolution show 14 Lya absorbers, 6 clustered at cz =16,100-18,500 km/s. ORFEUS claimed LyC absorption at z = 0.056 with N(HI) =(2-5)x10^16 cm^-2, while our Lya data suggest N(HI) = (3-10)x10^14 cm^-2.Higher columns are possible if the Lya line core at 17,000 +/- 50 km/s containsnarrow HI components. We identify the Lya cluster with a group of five HIgalaxies offset by 400-800 kpc from the sightline. The two strongest absorptionfeatures cover the same velocity range as the HI emission in the two galaxiesclosest to the line of sight. If the Lya is associated with these galaxies,they must have huge halos of highly turbulent, mostly ionized gas. The Lyaabsorption could also arise from an extended sheet of intragroup gas, or fromsmaller primordial clouds and halos of dwarf galaxies. We see no absorptionfrom SiIII 1206, CIV 1548, or DI Lya. Photoionization models yield limits of(Si/H) < 0.003 solar, (C/H) < 0.005 solar, (D/H) < 2.8x10^-4 (4 sigma) if N(HI)= 2x10^16 cm^-2. The limits increase to 0.023 solar and D/H < 2.8x10^-3 ifN(HI) = 2x10^15 cm^-2. The data suggest that the IGM in this group has not beenenriched to the levels suggested by X-ray studies of intracluster gas and thatthese absorbers could be primordial gas clouds.Comment: 34 pages LATEX, 8 figures (.ps), accepted for Astron. J. Vol. 116, Nov. 199