AN HST/COS SURVEY OF THE LOW REDSHIFT INTERGALACTIC MEDIUM. I. SURVEY, METHODOLOGY, AND OVERALL RESULTS
We use high quality, medium resolution Hubble Space Telescope/Cosmic Origins Spectrograph (HST/COS) observations of 82 UV bright active galactic nuclei (AGNs) at redshifts z(AGN) \textless 0.85 to construct the largest survey of the low redshift intergalactic medium (IGM) to date: 5138 individual extragalactic absorption lines in H I and 25 different metal ion species grouped into 2611 distinct redshift systems at z(abs) \textless 0.75 covering total redshift pathlengths Delta z(HI) 21.7 and Delta z(O VI) 14.5. Our semi automated line finding and measurement technique renders the catalog as objectively defined as possible. The cumulative column density distribution of H I systems can be parametrized dN \textgreater N)/dz C 14 (N/10(14) cm( 2) (beta 1) with C 14 25 1 and beta 1.65 0.02. This distribution is seen to evolve both in amplitude, C 14 infinity (1+z)(2.3 0.1) and slope beta(z) 1.75 0.31 z for z \textless= 0.47. We observe metal lines in 418 systems, and find that the fraction of IGM absorbers detected in metals is strongly dependent on N H I. The distribution of O VI absorbers appears to evolve in the same sense as the Ly alpha forest. We calculate contributions to Omega(b) from different components of the low z IGM and determine the Ly alpha decrement as a function of redshift. IGM absorbers are analyzed via a two point correlation function in velocity space. We find substantial clustering of H I absorbers on scales of Delta v 50 300 km s( 1) with no significant clustering at Delta(v) greater than or similar to 1000 km s( 1) Splitting the sample into strong and weak absorbers, we see that most of the clustering occurs in strong, N H I greater than or similar to 10(13.5) cm( 2) metal bearing IGM systems. The full catalog of absorption lines and fully reduced spectra is available via the Mikulski Archive for Space Telescopes (MAST) as a high level science product at http:/archive.stsci.edu/prepds/igm/