A Cluster of Low-Redshift LYalpha Clouds toward PKS 2155-304. I. Limits on Metals and D/H

Abstract

We report observations from the Hubble Space Telescope (HST) and the VLA on the galactic environment, metallicity, and D/H in strong low-redshift Lyalpha absorption systems toward the bright BL Lac object PKS 2155-304. These studies are intended to clarify the origin and chemical evolution of gas at large distances from galaxies. With GHRS/G160M data at ~20 km s^-1 resolution, we detect a total of 14 Lyalpha absorbers, six of them clustered between cz = 16,100 and 18,500 km s^-1. Although ORFEUS studies claimed Lyman continuum (Lyc) absorption at z ~ 0.056 with N(H I) = (2-5) x 10^16 cm^-2, the Lyalpha data suggest a range, N(H I) = (3-10) x 10^14 cm^-2. Even higher columns, needed for consistency with the ORFEUS Lyc results, are possible if the Lyalpha line core at 17,000 +/- 50 km s^-1 contains narrow H I components. We identify the Lyalpha cluster with a group of five H I galaxies offset by 400-800 h^-1_75 kpc from the sight line. The two strongest absorption features cover the same velocity range as the H I emission in the two galaxies closest to the line of sight. If the Lyalpha is associated with these galaxies, they must have huge halos (400-500 h^-1_75 kpc) of highly turbulent, mostly ionized gas. The Lyalpha absorption could also arise from an extended sheet of intragroup gas, or from smaller primordial clouds and halos of dwarf galaxies. We see no absorption from Si III lambda1206, C IV lambda1548, or deuterium Lyalpha at the expected positions of the strongest Lyalpha absorbers. Photoionization models yield (4 sigma) limits of (Si/H) <= 0.003(Si/H)_⊙, (C/H) <= 0.005(C/H)_⊙, and (D/H) <= 2.8 x 10^-4 if N(H I) has the ORFEUS value of 2 x 10^16 cm^-2. The limits increase to 0.023 solar metallicity and D/H <= 2.8 x 10^-3 if N(H I) is only 2 x 10^15 cm^-2. These limits can be improved with further studies by HST/STIS and measurements of the Lyc and higher Lyman series absorption by FUSE. However, the current data suggest that the intergalactic gas in this group has not been enriched to the levels suggested by X-ray studies of intracluster gas. Because of their low metallicity and large distance from galaxies, these absorbers could be primordial gas clouds.