AN HST/COS SURVEY OF THE LOW-REDSHIFT INTERGALACTIC MEDIUM. I. SURVEY, METHODOLOGY, AND OVERALL RESULTS*

Abstract

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  < 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  < 0.75 covering total redshift pathlengths Δ z H i  = 21.7 and Δ 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  =  , with C 14  = 25 ± 1 and β  = 1.65 ± 0.02. This distribution is seen to evolve both in amplitude, , and slope β ( z ) = 1.75–0.31 z for z  ≤ 0.47. We observe metal lines in 418 systems, and find that the fraction of IGM absorbers detected in metals is strongly dependent on . The distribution of O vi absorbers appears to evolve in the same sense as the Ly α forest. We calculate contributions to Ω b from different components of the low- z IGM and determine the Ly α 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 Δ v  = 50–300 km s −1 with no significant clustering at Δ v  ≳ 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  ≳ 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/ .