The COS CGM Compendium (CCC). I: Survey Design and Initial Results
  We present a neutral hydrogen-selected absorption-line survey of gas with HI
column densities 15<log N(HI)<19 at z<1 using the Cosmic Origins Spectrograph
on the Hubble Space Telescope. Our main aim is to determine the metallicity
distribution of these absorbers. Our sample consists of 224 absorbers selected
on the basis of their HI absorption strength. Here we discuss the properties of
our survey and the immediate empirical results. We find singly and doubly
ionized metal species and HI typically have similar velocity profiles, implying
they probe gas in the same or similar environments. The column density ionic
ratios (e.g., CII/CIII, OI/CII) indicate the gas in these absorbers is largely
ionized, and the ionization conditions are quite comparable across the sampled
N(HI) range. The Doppler parameters of the HI imply T<50,000 K on average,
consistent with the gas being photoionized. The MgII column densities span >2
orders of magnitude at any given N(HI), indicating a wide range of
metallicities (from solar to <1/100 solar). In the range 16.2<log N(HI)<17,
there is a gap in the N(MgII) distribution corresponding to gas with ~10% solar
metallicity, consistent with the gap seen in the previously identified bimodal
metallicity distribution in this column density regime. Less than 3% of the
absorbers in our sample show no detectable metal absorption, implying
truly-pristine gas at z<1 is uncommon. We find [FeII/MgII] = -0.4+/-0.3, and
since alpha-enhancement can affect this ratio, dust depletion is extremely
mild.
