Characterizing the Circumgalactic Medium of Nearby Galaxies with HST/COS and HST/STIS Absorption-line Spectroscopy. II. Methods and Models

We present basic data and modeling for a survey of the cool, photoionized circumgalactic medium (CGM) of low-redshift galaxies using far-UV QSO absorption-line probes. This survey consists of “targeted” and “serendipitous” CGM subsamples, originally described in Stocke et al. (Paper I). The targeted subsample probes low-luminosity, late-type galaxies at z < 0.02 with small impact parameters ( 〈 ρ 〉 = 71 kpc), and the serendipitous subsample probes higher luminosity galaxies at z ≲ 0.2 with larger impact parameters ( 〈 ρ 〉 = 222 kpc). Hubble Space Telescope and FUSE UV spectroscopy of the absorbers and basic data for the associated galaxies, derived from ground-based imaging and spectroscopy, are presented. We find broad agreement with the COS-Halos results, but our sample shows no evidence for changing ionization parameter or hydrogen density with distance from the CGM host galaxy, probably because the COS-Halos survey probes the CGM at smaller impact parameters. We find at least two passive galaxies with H i and metal-line absorption, confirming the intriguing COS-Halos result that galaxies sometimes have cool gas halos despite no on-going star formation. Using a new methodology for fitting H i absorption complexes, we confirm the CGM cool gas mass of Paper I, but this value is significantly smaller than that found by the COS-Halos survey. We trace much of this difference to the specific values of the low- z metagalactic ionization rate assumed. After accounting for this difference, a best-value for the CGM cool gas mass is found by combining the results of both surveys to obtain log ( M / M ⊙ ) = 10.5 ± 0.3 , or ∼30% of the total baryon reservoir of an L ≥ L * , star-forming galaxy.