We describe the Sloan

The fourth generation of the Sloan

The fourth generation of the Sloan

The ionizing fluxes from quasars and other active galactic nuclei (AGN) are critical for interpreting their emission-line spectra and for photoionizing and heating the intergalactic medium (IGM). Using far-ultraviolet spectra from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST), we directly measure the rest-frame ionizing continua and emission lines for 159 AGN at redshifts 0.001 < z AGN < 1.476 and construct a composite spectrum from 475-1875Å. We identify the underlying AGN continuum and strong EUV emission lines from ions of oxygen, neon, and nitrogen after masking out absorption lines from the H I Lyα forest, 7 Lyman-limit systems (N HI ≥ 10 17.2 cm −2 ) and 214 partial Lyman-limit systems (15.0 < log N HI < 17.2). The 159 AGN exhibit a wide range of FUV/EUV spectral shapes, F ν ∝ ν αν typically with −2 ≤ α ν ≤ 0 and no discernible continuum edges at 912Å (H I) or 504Å (He I). The composite rest-frame continuum shows a gradual break at λ br ≈ 1000Å, with mean spectral index α ν = −0.83 ± 0.09 in the FUV (1200-2000Å) steepening to α ν = −1.41±0.15 in the EUV (500-1000Å). We discuss the implications of the UV flux turnovers and lack of continuum edges for the structure of accretion disks, AGN mass inflow rates, and luminosities relative to Eddington values.

We present basic data and modeling for a survey of the cool, photo-ionized Circum-Galactic 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. (2013, Paper 1). 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). HST 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 1, but this value is significantly smaller than found by the COS-Halos survey. We trace much of this difference to the specific values of the low-z meta-galactic 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.