J/ApJ/817/111 HST/COS survey of z<0.9 AGNs. I. (Danforth+, 2016) ================================================================================ An HST/COS survey of the low-redshift intergalactic medium. I. Survey, methodology, and overall results. Danforth C.W., Keeney B.A., Tilton E.M., Shull J.M., Stocke J.T., Stevans M., Pieri M.M., Savage B.D., France K., Syphers D., Smith B.D., Green J.C., Froning C., Penton S.V., Osterman S.N. =2016ApJ...817..111D (SIMBAD/NED BibCode) ================================================================================ ADC_Keywords: Active gal. nuclei ; Spectra, ultraviolet ; Redshifts ; Surveys Keywords: cosmological parameters; cosmology: observations; intergalactic medium; quasars: absorption lines; surveys Abstract: 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. Description: COS is the fourth-generation UV spectrograph on board HST and is optimized for medium-resolution (R~18000, {Delta}v~17km/s) spectroscopy of point sources in the 1135-1800{AA} band. To constitute our survey, we selected 82 AGN sight lines from the archive which met the selection criteria. Most of the AGNs observed in Cycles 18-20 under the Guaranteed Time Observation programs (GTO; PI-Green) are included, along with numerous archival data sets collected under various Guest Observer programs. Observational and programatic details are presented in Table 2; see also section 2.1. File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file table1.dat 89 82 HST/COS sight lines table2.dat 77 89 COS observation details linelist.dat 92 11844 HST/COS linelists for all the sightlines systems.dat 106 4930 HST/COS systems for all the sightlines igm/* 0 88 Individual subdirectories for each sightline -------------------------------------------------------------------------------- Description of file: The "igm" repertory downloaded on http://archive.stsci.edu/prepds/igm/ and available on the FTP, is described in detail in section 3. See also: J/A+A/590/A68 : AGN data and absorption-line measurements (Richter+, 2016) J/ApJ/814/40 : Nearby galaxy filaments with UV obs. (Wakker+, 2015) J/MNRAS/449/3263 : Extended galaxy halo gas through HI and OVI (Johnson+, 2015) J/ApJ/759/112 : The UV low-redshift intergalactic medium (Tilton+, 2012) J/ApJ/740/91 : Ly{alpha} & OVI in galaxies around QSOs (Prochaska+, 2011) J/ApJ/736/42 : HST QSO Catalog (Ribaudo+, 2011) J/ApJ/734/65 : CI radial velocities with HST/STIS (Jenkins+, 2011) J/ApJ/710/613 : Broad HI absorbers (Danforth+, 2010) J/ApJ/701/1219 : Galaxy survey in 3 QSO fields (Chen+, 2009) J/ApJS/177/39 : Survey of low-redshift OVI absorbers (Tripp+, 2008) J/ApJ/679/194 : Low-z intergalactic medium. III. (Danforth+, 2008) J/A+A/458/427 : Lyman alpha absorbers in 0.5