The low-z intergalactic medium.: II.: Lyβ, OVI, and CIII forest

We present the results of a large survey of H (I), O (VI), and C (III) absorption lines in the low- redshift (z < 0.3) intergalactic medium (IGM). We begin with 171 strong Ly alpha absorption lines (W-lambda >= 80 m angstrom) in 31 AGN sight lines studied with the Hubble Space Telescope and measure corresponding absorption from higher order Lyman lines with FUSE. Higher order Lyman lines are used to determine NH (I) and bH (I) accurately through a curve-of-growth (COG) analysis. We find that the number of Hi absorbers per column density bin is a power-law distribution, dN/dN(HI) proportional to N-beta, with beta(H) (I) = 1.68 +/- 0: 11. We made 40 detections of O (VI) lambda lambda 1032, 1038 and 30 detections of C (III) lambda 977 out of 129 and 148 potential absorbers, respectively. The column density distribution of C (III) absorbers has beta(C III) 1.68 +/- 0.04, similar to beta(H) (I) but not as steep as beta(O) (VI) 2.2 +/- 0.1. From the absorption-line frequency, dN(C) (III)/dz = 12(-2)(+3) for W-lambda(C (III)) > 30 m angstrom, we calculate a typical IGM absorber size r(0) similar to 400 kpc, similar to scales derived by other means. The COG-derived b-values show that H i samples material with T < 10(5) K, incompatible with a hot IGM phase. By calculating a grid of CLOUDY models of IGM absorbers with a range of collisional and photoionization parameters, we find it difficult to simultaneously account for the O (VI) and C (III) observations with a single phase. Instead, the observations require a multiphase IGM in which H (I) and C (III) arise in photoionized regions, while O vi is produced primarily through shocks. From the multiphase ratio N-H (I)/N-C (III), we infer the IGM metallicity to be Z(C) = 0.12 Z(circle dot), similar to our previous estimate of Z(O) = 0.09 Z(circle dot) from O (VI).