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We measure the distribution of carbon in the intergalactic medium as a function of redshift z and overdensity δ. Using a hydrodynamical simulation to link the H I absorption to the density and temperature of the absorbing gas, and a model for the UV background radiation, we convert ratios of C IV to H I pixel optical depths into carbon abundances. For the median metallicity this technique was described and tested in Paper I of this series. Here we generalize it to reconstruct the full probability distribution of the carbon abundance and apply it to 19 high-quality quasar absorption spectra. We find that the carbon abundance is spatially highly inhomogeneous and is well-described by a lognormal distribution for fixed δ and z. Using data in the range log δ = −0.5 − 1.8 and z = 1.8 − 4.1, and a renormalized version of the Haardt & Madau (2001) model for the UV background radiation from galaxies and quasars, we measure a median metallicity of [C/H] = −3.47 +0.07 −0.06 + 0.08 +0.09 −0.10 (z − 3) + 0.65 +0.10 −0.14 (log δ − 0.5) and a lognormal scatter of σ([C/H]) = 0.76 +0.05 −0.08 + 0.02 +0.08 −0.12 (z − 3) − 0.23 +0.09 −0.07 (log δ − 0.5). Thus, we find significant trends with overdensity, but no evidence for evolution. These measurements imply that gas in this density range accounts for a cosmic carbon abundance of [C/H] = −2.80 ± 0.13 (Ω C ≈ 2 × 10 −7 ), with no evidence for evolution. The dominant source of systematic error is the spectral shape of the UV background, with harder spectra yielding higher carbon abundances. While the systematic errors due to uncertainties in the spectral hardness may exceed the quoted statistical errors for δ < 10, we stress that UV backgrounds that differ significantly from our fiducial model give unphysical results. The measured lognormal scatter is strictly independent of the spectral shape, provided the background radiation is uniform. We also present measurements of the C III/C IV ratio (which rule out temperatures high enough for collisional ionization to be important for the observed C IV) and of the evolution of the effective Lyα optical depth.

At redshifts z >~ 2, most of the baryons reside in the smooth intergalactic medium which is responsible for the low column density Lyman-alpha forest. This photoheated gas follows a tight temperature-density relation which introduces a cut-off in the distribution of widths of the Lyman-alpha absorption lines (b-parameters) as a function of column density. We have measured this cut-off in a sample of nine high resolution, high signal-to-noise quasar spectra, and determined the thermal evolution of the intergalactic medium in the redshift range 2.0-4.5. At redshift z ~ 3, the temperature at the mean density shows a peak and the gas becomes nearly isothermal. We interpret this as evidence for the reionization of HeII.Comment: Accepted for publication in MNRAS, 11 pages, 7 figures. Revised version shortened, discussion clarified, conclusions unchange

We have completed an optical spectroscopic survey of the nuclear regions pc) of a large (r [ 200 sample of nearby galaxies. Although the main objectives of the survey are to search for low-luminosity active galactic nuclei and to quantify their luminosity function, the database can be used for a variety of other purposes. This paper presents measurements of the spectroscopic parameters for the 418 emissionline nuclei, along with a compilation of the global properties of all 486 galaxies in the survey. Stellar absorption generally poses a serious obstacle to obtaining accurate measurement of emission lines in nearby galactic nuclei. We describe a procedure for removing the starlight from the observed spectra in an efficient and objective manner. The main parameters of the emission lines (intensity ratios, Ñuxes, proÐle widths, and equivalent widths) are measured and tabulated, as are several stellar absorption-line and continuum indices useful for studying the stellar population. Using standard nebular diagnostics, we determine the probable ionization mechanisms of the emission-line objects. The resulting spectral classiÐcations provide extensive information on the demographics of emission-line nuclei in the nearby regions of the universe. This new catalog contains over 200 objects showing spectroscopic evidence for recent star formation and an equally large number of active galactic nuclei, including 46 that show broad Ha emission. These samples will serve as the basis of future studies of nuclear activity in nearby galaxies.

We use the sample of emission-line nuclei derived from a recently completed optical spectroscopic survey of nearby galaxies to quantify the incidence of local (z ≈ 0) nuclear activity. Particular attention is paid to obtaining accurate measurements of the emission lines and reliable spectral classifications. The resulting data base contains the largest collection of star-forming nuclei and active galactic nuclei (AGNs) currently known for nearby galaxies. It consists of 420 emission-line nuclei detected from a nearly complete, magnitude-limited sample of 486 galaxies with B T ≤ 12.5 mag and declination > 0 • ; the equivalent-width detection limit of the brightest emission line, usually Hα, is ∼0.25Å.Consistent with previous studies, we find detectable amounts of ionized gas in the central few hundred parsecs of most (86%) galaxies; emission lines are present in essentially every spiral galaxy and in a large fraction of ellipticals and lenticulars. Based on their narrow-line spectra, half of the objects can be classified as H II or star-forming nuclei and the other half as some form of AGN, of which we distinguish three classes -Seyfert nuclei, low-ionization nuclear emission-line regions (LINERs), and transition objects which we assume to be composite LINER/H II-nucleus systems. The population of AGNs consequently is very large; approximately 43% of the galaxies in our survey can be regarded as "active," although, for a number of reasons, this