2018ApJ...866...33L

We present a neutral hydrogen-selected absorption-line survey of gas with H I column densities 15< logN_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 H I absorption strength. Here we discuss the properties of our survey and the immediate empirical results. We find singly and doubly ionized metal species, and H I typically have similar velocity profiles, implying they probe gas in the same or similar environments. The ionic ratios (e.g., N_CII/N_CIII, N_OI/N_CII) indicate that 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 H I imply T≲5×10⁴ K on average, consistent with the gas being photoionized. The Mg II 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 of 16.2≲logN_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 that truly pristine gas at z≲1 is uncommon. We find <[Fe II/Mg II]>=-0.4±0.3, and since α-enhancement can affect this ratio, dust depletion is extremely mild.