A CLUSTER OF LOW-REDSHIFT LY-ALPHA CLOUDS TOWARD PKS 2155-304 - I - LIMITS ON METALS AND D H/

We report observations from the Hubble Space Telescope (HST) and the V LA on the galactic environment, metallicity, and D/H in strong low-red shift Ly alpha absorption systems toward the bright BL Lac object PKS 2155-304. These studies are intended to clarify the origin and chemica l evolution of gas at large distances from galaxies. With GHRS/G160M d ata at similar to 20 km s(-1) resolution, we detect a total of 14 Ly a lpha absorbers, six of them clustered between ct = 16,100 and 18,500 k m s(-1). Although ORFEUS studies claimed Lyman continuum (Lyc) absorpt ion at z approximate to 0.056 with N(H I) = (2-5) x 10(16) cm(-2), the Ly alpha data suggest a range, N(H I) = (3-10) x 10(14) cm(-2). Even higher columns, needed for consistency with the ORFEUS Lye results, ar e possible if the Ly alpha line core at 17,000 +/- 50 km s(-1) contain s narrow H I components. We identify the Ly alpha cluster with a group of five H I galaxies offset by 400-800 h(75)(-1) kpc from the sight l ine. The two strongest absorption features cover the same velocity ran ge as the H r emission in the two galaxies closest to the line of sigh t. If the Ly alpha is associated with these galaxies, they must have h uge halos (400-500 h(75)(-1) kpc) of highly turbulent, mostly ionized gas. The Ly alpha absorption could also arise from an extended sheet o f intragroup gas, or from smaller primordial clouds and halos of dwarf galaxies. We see no absorption from Si III lambda 1206, C IV lambda 1 548, or deuterium Ly alpha at the expected positions of the strongest Ly alpha absorbers. Photoionization models yield (4 sigma) limits of ( Si/H) less than or equal to 0.003(Si/H)., (C/H) less than or equal to 0.005(C/H)., and (D/H) less than or equal to 2.8 x 10(-4) if N(H I) ha s the ORFEUS value of 2 x 10(16) cm(-2). The limits increase to 0.023 solar metallicity and D/H less than or equal to 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 Lye and higher Lyman serie s absorption by FUSE. However, the current data suggest that the inter galactic gas in this group has not been enriched to the levels suggest ed by X-ray studies of intracluster gas. Because of their low metallic ity and large distance from galaxies, these absorbers could be primord ial gas clouds.