The Low-z Intergalactic Medium. I. OVI Baryon Census

Charles Danforth & J. Michael Shull 2004, ApJ, 624, 555


Intergalactic 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 (logNOVI=12.8-14.4) than seen in HI (logNHI=13.0-16.0). The co-existence 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 OmegaWHIMh70=0.0030+-0.0005, assuming (O/H) of 10% solar metallicity and an ionization fraction fOVI=0.2. Thus, gas in the WHIM at 105-6 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.

The Low-z Intergalactic Medium. II. LyB, OVI, and CIII Forest

Danforth, Shull, Rosenberg, & Stocke 2004, submitted to ApJ, astro-ph/0508656


We present the results of a large survey of HI, OVI, and CIII absorption lines in the low-redshift (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 higher-order Lyman lines with FUSE. Higher-order Lyman lines are used to determine NHI and bHI 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/dNHI=NHI-beta, with betaHI=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 betaCIII=1.68+-0.04, similar to betaHI but not as steep as betaOVI=2.1+-0.1. From the absorption-line frequency, dNCIII/dz=12+3-2 for W>30 mA, we calculate a typical IGM absorber size r0~400 kpc. The COG-derived b-values show that HI samples material with T less than 105 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 NHI/NCIII, we infer the IGM metallicity to be ZC=0.12Zsun, similar to our previous estimate of ZO=0.09Zsun from OVI.

Charles Danforth
Last modified: Thu Sep 1 08:58:59 MDT 2005