An HST/COS Survey of the Low-Redshift IGM. I. Survey, Methodology, & Overall Results

Abstract

We use high-quality, medium-resolution {\it Hubble Space Telescope}/Cosmic Origins Spectrograph (\HST/COS) observations of 82 UV-bright AGN at redshifts $z_{AGN}<0.85$ to construct the largest survey of the low-redshift intergalactic medium (IGM) to date: 5343 individual extragalactic absorption lines in HI and 25 different metal-ion species grouped into 2610 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 $dN(>N)/dz=C_{14}(N/10^{14}c{m}^{-2}{)}^{-(\beta -1)}$, with $C_{14}=25\pm 1$ and $\beta =1.65\pm 0.02$. This distribution is seen to evolve both in amplitude, $C_{14}\sim (1+z{)}^{2.0\pm 0.1}$, and slope $\beta (z)=1.73-0.26z$ for $z<0.47$. We observe metal lines in 427 systems, and find that the fraction of IGM absorbers detected in metals is strongly dependent on N_{HI}. The distribution of OVI absorbers appear to evolve in the same sense as the Lya forest. We calculate contributions to ${\Omega }_b$ from different components of the low-$z$ IGM and determine the Lya decrement as a function of redshift. IGM absorbers are analyzed via a two-point correlation function (TPCF) in velocity space. We find substantial clustering of \HI\ absorbers on scales of $\Delta v=50-300$ km/s with no significant clustering at $\Delta v>1000$ km/s. Splitting the sample into strong and weak absorbers, we see that most of the clustering occurs in strong, $N_{HI}>10^{13.5}c{m}^{-2}$, metal-bearing IGM systems. The full catalog of absorption lines and fully-reduced spectra is available via MAST as a high-level science product at http://archive.stsci.edu/prepds/igm/.

This is the accepted version (v3) of the paper. Previous versions (July 2015 and Feb. 2014) should be replaced by this one. In particular, please note that the associated MAST high-level-science product has been updated to reflect the of the final state of the paper. It is available at: http://archive.stsci.edu/prepds/igm/

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