 | Abstract
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.
|
 | Abstract
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.
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