2016ApJ...817..111D

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 Δz_ H i_= 21.7 and Δz_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 d N(>N)/dz = C₁₄(N/10¹⁴/cm2)^–(β–1), with C₁₄= 25±1 and β = 1.65±0.02. This distribution is seen to evolve both in amplitude, C₁₄ ∝ (1 + z)^2.3±0.1, 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 N_H1. The distribution of O vi absorbers appears to evolve in the same sense as the Lyα forest. We calculate contributions to Ω_b from different components of the low-z IGM and determine the Lyα 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 Δv = 50-300 km/s with no significant clustering at Δv ≳ 1000 km/s. Splitting the sample into strong and weak absorbers, we see that most of the clustering occurs in strong, N_H i_≳ 10^13.5/cm2, 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/.