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Description
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 HI and 25 different metal-ion species grouped into 2611 distinct redshift systems at z_abs_<0.75 covering total redshift pathlengths {Delta}z_HI_=21.7 and {Delta}z_OVI_=14.5. Our semi-automated line-finding and measurement technique renders the catalog as objectively defined as possible. The cumulative column density distribution of HI systems can be parametrized d N(>N)/dz=C_14_(N/10^14^/cm2)^-({beta}-1)^, with C_14_=25+/-1 and {beta}=1.65+/-0.02. This distribution is seen to evolve both in amplitude, C_14_{propto}(1+z)^2.3+/-0.1^, and slope {beta}(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 OVI 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 HI absorbers on scales of {Delta}v=50-300km/s with no significant clustering at {Delta}v>~1000km/s. Splitting the sample into strong and weak absorbers, we see that most of the clustering occurs in strong, N_HI_>~10^13.5^/cm2, metal-bearing IGM systems.
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