The Low-z Intergalactic Medium. II. Lyb, O VI, and C III 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 Lya absorption lines (W sub(l) . 80 mAe) 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 N super(H I) and b super(H I) accurately through a curve-of-growth (COG) analysis. We find that the number of H I absorbers per column density bin is a power-law distribution, dN/dN super(H I) 8 N sub(H I) super(-b), with b super(H I) = 1.68 c 0.11. We made 40 detections of O VI ll1032, 1038 and 30 detections of C III l977 out of 129 and 148 potential absorbers, respectively. The column density distribution of C III absorbers has b super(C III) = 1.68 c 0.04, similar to b super(H I) but not as steep as b super(O V I) = 2.2 c 0.1. From the absorption-line frequency, dN super(C III)/dz = 12 sub(-2) super(+3) for W sub(l)(C super(III)) > 30 mAe, we calculate a typical IGM absorber size r sub(0) 6 400 kpc, similar to scales derived by other means. The COG-derived b-values show that HI samples material with T< 10 super(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 super(H I)/N super(C III), we infer the IGM metallicity to be Z sub(C) = 0.12 Z sub(z), similar to our previous estimate of Z sub(O) = 0.09 Z sub(z) from O VI.