An HST/COS Survey of the Low-Redshift IGM. I. Survey, Methodology, &
  Overall Results
  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
$dN(>N)/dz=C_{14}(N/10^{14} cm^{-2})^{-(\beta-1)}$, with $C_{14}=25\pm1$ and
$\beta=1.65\pm0.02$. This distribution is seen to evolve both in amplitude,
$C_{14}\sim(1+z)^{2.0\pm0.1}$, and slope $\beta(z)=1.73-0.26 z$ 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, $N_{HI}>10^{13.5} cm^{-2}$, 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/.
