We use high-quality, medium-resolution Hubble Space Telescope/Cosmic Origins Spectrograph (HST/COS) observations of 82 UV-bright AGN 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 H I and 25 different metal-ion species grouped into 2611 distinct redshift systems at z abs < 0.75 covering total redshift pathlengths ∆z HI = 21.7 and ∆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 H I systems can be parametrized dN (> N )/dz = C 14 (N/10 14 cm −2 ) −(β−1) , with C 14 = 25 ± 1 and β = 1.65 ± 0.02. This distribution is seen to evolve both in amplitude, C 14 ∝ (1 + z) 2.3±0.1 , and slope β(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 HI . The distribution of O VI absorbers appear to evolve in the same sense as the Lyα forest. We calculate contributions to Ω b from different components of the low-z IGM and determine the Lyα decrement as a function of redshift. IGM absorbers are analyzed via a two-point correlation function in velocity space. We find substantial clustering of H I absorbers on scales of ∆v = 50 − 300 km s −1 with no significant clustering at ∆v 1000 km s −1 . Splitting the sample into strong and weak absorbers, we see that most of the clustering occurs in strong, N HI 10 13.5 cm −2 , metal-bearing IGM systems. The full catalog of absorption lines and fully-reduced spectra is available via the Mikulski Archive for Space Telescopes (MAST) as a high-level science product at

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.

Recent suggestions of a "photon underproduction crisis" (Kollmeier et al. 2014) have generated concern over the intensity and spectrum of ionizing photons in the metagalactic ultraviolet background (UVB). The balance of hydrogen photoionization and recombination determines the opacity of the low-redshift intergalactic medium (IGM). We calibrate the hydrogen photoionization rate (Γ H ) by comparing Hubble Space Telescope spectroscopic surveys of the low-redshift column density distribution of H I absorbers and the observed (z < 0.4) mean Lyα flux decrement, D A = (0.014)(1 + z) 2.2 , to new cosmological simulations. The distribution, f (N HI , z) ≡ d 2 N /d(log N HI )dz, is consistent with an increased UVB that includes contributions from both quasars and galaxies. Our recommended fit, Γ H (z) = (4.6 × 10 −14 s −1 )(1 + z) 4.4 for 0 < z < 0.47, corresponds to unidirectional LyC photon flux Φ 0 ≈ 5700 cm −2 s −1 at z = 0. This flux agrees with observed IGM metal ionization ratios (C III/C IV and Si III/Si IV) and suggests a 25-30% contribution of Lyα absorbers to the cosmic baryon inventory. The primary uncertainties in the low-redshift UVB are the contribution from massive stars in galaxies and the LyC escape fraction (f esc ), a highly directional quantity that is difficult to constrain statistically. We suggest that both quasars and low-mass starburst galaxies are important contributors to the ionizing UVB at z < 2. Their additional ionizing flux would resolve any crisis in photon underproduction.

We present a comprehensive catalog of ultraviolet (HST /STIS and FUSE ) absorbers in the lowredshift IGM at z < 0.4. The catalog draws from the extensive literature on IGM absorption, and it reconciles discrepancies among previous catalogs through a critical evaluation of all reported absorption features in light of new HST /COS data. We report on 746 H I absorbers down to a rest-frame equivalent width of 12 mÅ over a maximum redshift path length ∆z = 5.38. We also confirm 111 O VI absorbers, 29 C IV absorbers, and numerous absorption features due to other metal ions. We characterize the distribution of absorber line frequency as a function of column density as a power law, dN /dz ∝ N −β , where β = 2.08 ± 0.12 for O VI and β = 1.68 ± 0.03 for H I. Utilizing a more sophisticated accounting technique than past work, the catalog accounts for ∼ 43% of the baryons: 24 ± 2% in the photoionized Lyα forest and 19 ± 2% in the WHIM as traced by O VI. We discuss the large systematic effects of various assumed metallicities and ionization states on these calculations, and we implement recent simulation results in our estimates.

We have analyzed Chandra LETG and XMM-Newton RGS spectra towards the z = 0.177 quasar PG 1116+215, a sightline that is rendered particularly interesting by the HST detection of several O VI and H I broad Lyman-α absorption lines that may be associated with the warm-hot intergalactic medium.We performed a search for resonance Kα absorption lines from O VII and O VIII at the redshifts of the detected far-ultraviolet lines. We detected an absorption line in the Chandra spectra at 5.2 σ confidence level at wavelengths corresponding to O VIII Kα at z = 0.0911 ± 0.0004 ± 0.0005 (statistical followed by systematic error). This redshift is within 3 σ of that of a H I broad Lyman-α of b 130 km/s (corresponding to a temperature of log T (K) 6.1) at z = 0.09279 ± 0.00005. We have also analyzed the available XMM-Newton RGS data towards PG 1116+215. Unfortunately, the XMM-Newton data are not suitable to investigate this line because of instrumental features at the wavelenghts of interest. At the same redshift, the Chandra and XMM-Newton spectra have O VII Kα absorption line features of significance 1.5 σ and 1.8 σ, respectively.We also analyzed the available SDSS spectroscopic galaxy survey data towards PG 1116+215 in the redshift range of interest. We found evidence for a galaxy filament that intersect the PG 1116+215 sightline and additional galaxy structures that may host WHIM. The H I BLA and the O VIII Kα absorbers are within a few Mpc of the filament (assuming that redshifts track Hubble flow distances) or consistent with gas accreting onto the filament from either direction relative to the sightline with velocities of a few ×100 km/s.The combination of HST, Chandra, XMM-Newton and SDSS data indicates that we have likely detected a multi-temperature WHIM at z 0.091 − 0.093 towards -2 -PG 1116+215. The O VIII Kα absorption line indicates gas at high temperature, log T (K) ≥ 6.4, with a total column density of order log N H (cm 2 ) ≥ 20 and a baryon overdensity δ b ∼ 100−1000 for sightline lengths of L = 1−10 Mpc. This detection highlights the importance of BLA absorption lines as possible signposts of high-temperature WHIM filaments.

The rest-frame ultraviolet (UV) spectra of active galactic nuclei (AGNs) are important diagnostics of both accretion disk physics and their contribution to the metagalactic ionizing UV background. Though the mean AGN spectrum is well characterized with composite spectra at wavelengths greater than 912 Å, the shorter-wavelength extreme-UV (EUV) remains poorly studied. In this third paper in a series on the spectra of AGNs, we combine 11 new spectra taken with the Cosmic Origins Spectrograph on the Hubble Space Telescope with archival spectra to characterize the typical EUV spectral slope of AGNs from λ rest ∼850 Å down to λ rest ∼425 Å. Parameterizing this slope as a power law, we obtain F ν ∝ν −0.72±0.26, but we also discuss the limitations and systematic uncertainties of this model. We identify broad emission features in this spectral region, including emission due to ions of O, Ne, Mg, and other species, and we limit the intrinsic He I 504 Å photoelectric absorption edge opacity to τ He I <0.047.

We present an ultraviolet spectroscopic survey of strong H I absorbers in the intergalactic medium, probing their evolution over the last 6-7 Gyr at redshifts 0.24 ≤ z ≤ 0.84. We measure column densities N HI (cm −2 ) from the pattern of Lyman-series absorption lines and flux decrement at the Lyman limit (LL) when available. We analyzed 220 H I absorbers in ultraviolet spectra of 102 active galactic nuclei (AGN) taken by the Cosmic Origins Spectrograph aboard the Hubble Space Telescope with G130M/G160M gratings (1134-1795Å). For 158 absorbers with log N HI ≥ 15, the mean frequency is dN /dz = 4.95 ± 0.39 over pathlength ∆z = 31.94 (0.24 ≤ z ≤ 0.84). We identify 8 Lyman Limit Systems (LLS, log N HI ≥ 17.2) and 54 partial systems (pLLS) with 16.0 ≤ log N HI < 17.2. Toward 159 AGN between 0.01 < z abs < 0.84 with ∆z ≈ 48, we find four damped Lyα absorbers (DLA) with (dN /dz) DLA = 0.083 +0.066 −0.040 at z = 0.18. The mean LLS frequency between z = 0.24 − 0.48 is (dN /dz) LLS = 0.36 +0.20 −0.13 fitted to N (z) = (0.25 +0.13 −0.09 )(1 + z) 1.14 . For 54 pLLS we find (dN /dz) pLLS = 1.69±0.23 at z = 0.39, a frequency consistent with gaseous halo sizes R ≈ 100h −1 kpc for (0.3−3L * ) galaxies. A maximum-likelihood analysis yields a distribution f (N, z) = C 0 N −β (1 + z) γ with β = 1.48 ± 0.05 and γ = 1.14 +0.88 −0.89 for 15 ≤ log N HI ≤ 17.5. The far-UV opacity gradient is dτ eff /dz ≈ (0.444)(1 + z) 1.14 over the range 15 ≤ log N HI ≤ 17, implying mean LyC optical depth τ eff ≈ 0.3 − 0.5 toward sources at z = 1 − 2.

Effective methods of measuring supermassive black hole masses in active galactic nuclei (AGN) are of critical importance to studies of galaxy evolution. While there has been much success obtaining masses through reverberation mapping, the extensive observing time required by this method has limited the practicality of applying it to large samples at a variety of redshifts. This limitation highlights the need to estimate these masses using single-epoch spectroscopy of ultraviolet emission lines. We use ultraviolet spectra of 44 AGN from HST /COS, IUE, and FUSE of the C IV λ1549, O VI λ1035, O III] λ1664, He II λ1640, C II λ1335, and Mg II λ2800 emission lines and explore their potential as tracers of the broad-line region and supermassive black hole mass. The higher S/N and better spectral resolution of the Cosmic Origins Spectrograph on Hubble Space Telescope resolves AGN intrinsic absorption and produces more accurate line widths. From these, we test the viability of mass-scaling relationships based on line widths and luminosities and carry out a principal component analysis based on line luminosities, widths, skewness, and kurtosis. At L 1450 ≤ 10 45 erg s −1 , the UV line luminosities correlate well with Hβ, as does the 1450Å continuum luminosity. We find that C IV, O VI, and Mg II can be used as reasonably accurate estimators of AGN black hole masses, while He II and C II are uncorrelated.