AbstractIntergalactic absorbers along lines of sight to distant quasars are a powerful diagnostic for the evolution and content of the intergalactic medium (IGM). In this study, we use the FUSE satellite to search 128 known Lya absorption systems at z greater than 0.15 toward 31 AGN for corresponding absorption from higher Lyman lines and the important metal ions OVI and CIII. We detect OVI in 52 systems over a smaller range of column density (logN_{OVI}=12.814.4) than seen in HI (logN_{HI}=13.016.0). The coexistence of OVI and HI suggests a multiphase IGM, with both warm neutral and hot ionized components. With improved OVI detection statistics, we find a steep distribution in OVI column density, dNdN~N^{2.1}, which suggests that numerous, weak OVI absorbers contain baryonic mass comparable to the rare strong absorbers. The total cosmological mass fraction is at least Omega_{WHIM}h_{70}=0.0030+0.0005, assuming (O/H) of 10% solar metallicity and an ionization fraction f_{OVI}=0.2. Thus, gas in the WHIM at 10^{56} K contributes at least 6.6+1.1% of the total baryonic mass at low redshift, a value 50% higher than previous estimates. Our survey is based on a large improvement in the number of OVI absorbers (52 vs. 10) and total redshift pathlength (Delta z=2.2 vs. Delta z=0.5) compared to earlier surveys.

AbstractWe present the results of a large survey of HI, OVI, and CIII absorption lines in the lowredshift (z less than 0.3) intergalactic medium (IGM). We begin with 171 strong Lyalpha absorption lines (W > 80 mA) in 31 AGN sight lines studied with the Hubble Space Telescope and measure corresponding absorption from higherorder Lyman lines with FUSE. Higherorder Lyman lines are used to determine N_{HI} and b_{HI} accurately through a curveofgrowth (COG) analysis. We find that the number of HI absorbers per column density bin is a powerlaw distribution, dN/dN_{HI}=N_{HI}^{beta}, with beta_{HI}=1.68+0.11. We made 40 detections of OVI 1032,1038 and 30 detections of CIII 977 out of 129 and 148 potential absorbers, respectively. The column density distribution of CIII absorbers has beta_{CIII}=1.68+0.04, similar to beta_{HI} but not as steep as beta_{OVI}=2.1+0.1. From the absorptionline frequency, dN_{CIII}/dz=12^{+3}_{2} for W>30 mA, we calculate a typical IGM absorber size r_{0}~400 kpc. The COGderived bvalues show that HI samples material with T less than 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 OVI and CIII observations with a single phase. Instead, the observations require a multiphase IGM in which HI and CIII arise in photoionized regions, while OVI is produced primarily through shocks. From the multiphase ratio N_{HI}/N_{CIII}, we infer the IGM metallicity to be Z_{C}=0.12Z_{sun}, similar to our previous estimate of Z_{O}=0.09Z_{sun} from OVI.
