68 research outputs found
The enrichment history of the intergalactic medium: O VI in Ly-alpha forest systems at redshift z ~ 2
A search for O VI at redshifts corresponding to Ly-alpha lines in the z_em ~
2.4 QSOs HE1122-1648 and HE2217-2818 reveals that a substantial fraction of
those with H I column densities log N(HI) > 14 (cm^{-2}) are highly ionized and
show some heavy element enrichment. If these two sight lines are typical, then
the O VI systems contain a cosmologically significant fraction of the baryons
and the metals in the universe. For most systems the temperatures derived from
the line widths are too low for collisional ionization to be responsible for
the O VI lines. Photoionization models with a substantial hard ultraviolet flux
can reproduce the observations for densities that are in good agreement with a
model assuming local, hydrostatic equilibrium and heavy element abundances in
the range ~ 10^{-3} - 10^{-2} solar. Photoionization by a UV flux much softer
than that predicted by Haardt & Madau (1996) for a background dominated by
quasars can be ruled out. Finally, we find one system with a very low H I
column density for which both photoionization and collisional ionization models
yield a metallicity close to solar and a density that is inconsistent with
gravitational confinement, unless the gas fraction is negligible.Comment: 16 pages, 16 figures, 7 tables. Accepted for publication in the
Astrophysical Journal. Minor change
The evolution of HI and CIV quasar absorption line systems at 1.9 < z < 3.2
We have investigated the distribution and evolution of ~3100 intergalactic HI
absorbers with HI column densities log N(HI) = [12.75, 17.0] at 1.9 < z < 3.2,
using 18 high resolution, high S/N quasar spectra obtained from the ESO
VLT/UVES archive. We used two sets of Voigt profile fitting analysis, one
including all the available high-order Lyman lines to obtain reliable HI column
densities of saturated lines, and another using only the Ly-alpha lines. There
is no significant difference between the results from the two fits. Combining
our results with literature data, the mean number density at 0 < z < 4 is not
well described by a single power law and strongly suggests that its evolution
slows down at z < 1.5 at the high and low column density ranges. We also
divided our entire HI absorbers at 1.9 < z < 3.2 into two samples, the
unenriched forest and the CIV-enriched forest, depending on whether HI lines
are associated with CIV at log N(CIV) > 12.2 within a given velocity range. The
entire HI column density distribution function (CDDF) can be described as the
combination of these two well-characterised populations which overlap at log
N(HI) ~ 15. At log N(HI) < 15, the unenriched forest dominates, showing a
similar power-law distribution to the entire forest. The CIV-enriched forest
dominates at log N(HI) > 15, with its distribution function proportional to
N(HI)^(-1.45). However, it starts to flatten out at lower N(HI), since the
enriched forest fraction decreases with decreasing N(HI). The deviation from
the power law at log N(HI) = [14, 17] shown in the CDDF for the entire HI
sample is a result of combining two different HI populations with a different
CDDF shape. The total HI mass density relative to the critical density is
Omega(HI) ~ 1.6 x 10^(-6) h^(-1), where the enriched forest accounts for ~40%
of Omega(HI).Comment: 26 pages, 20 figures, accepted for AA, in pres
Mapping Low-Density Intergalactic Gas: a Third Helium Lyman-alpha Forest
We present a new HST/STIS spectrum of the z=3.18 quasar PKS 1935-692 and
summarize the spectral features shortwards of 304A in the rest frame likely to
be caused by foreground HeII Lyman-alpha absorption. In accord with previous
results on two other quasars at similar redshifts, we demonstrate a correlation
with the HI Lyman-alpha forest absorption, and show that much of the helium
absorption is caused by a comparable quantity of more diffuse gas with
Omega~0.01, that is not detected in HI. The helium ionization zone around the
quasar is detected as well as a void seen in both HI and HeII. The properties
of the absorption are in broad agreement with those of the other quasars and
with models of the protogalactic gas distribution and ionization at this
redshift.Comment: 17 pages including 5 figures. As accepted for publication in The
Astronomical Journal (minor revisions
A large population of metal-rich, compact, intergalactic C IV absorbers - Evidence for poor small-scale metal mixing
We carried out a survey for high-metallicity C IV absorbers at redshift z ~
2.3 in the spectra of 9 high-quality quasar spectra. Using a novel analysis
technique, based on detections of C IV lines and automatically determined upper
limits on the column densities of H I, C III, N V, and O VI, we find a large
(dN/dz > 7) population of photo-ionized, compact (R ~ 10^2 pc), metal-rich (Z
>~ Z_solar) C IV clouds with moderate densities (n_H ~ 10^{-3.5} cm^{-3}),
properties that we show are robust with respect to uncertainties in the
ionization model. In particular, local sources of ionizing radiation,
overabundance of oxygen, departures from ionization equilibrium, and
collisional ionization would all imply more compact clouds. The clouds are too
small to be self-gravitating and pressure confinement is only consistent under
special conditions. We argue that the clouds are, in any case, likely to be
short-lived and we demonstrate that this implies that the clouds could easily
have been responsible for the transport of all metals that end up in the
intergalactic medium (IGM). When the clouds reach pressure equilibrium with the
general, photo-ionized IGM, the metals will still be concentrated in small
high-metallicity patches, but they will look like ordinary, low-metallicity
absorbers. We conclude that intergalactic metals are poorly mixed on small
scales and that nearly all of the IGM, and thus the universe, may therefore be
of primordial composition.Comment: Accepted for publication in MNRAS, 29 pages, 24 figures. Minor
change
Spatial fluctuations in the spectral shape of the UV background at 2<z<3 and the reionization of helium
The low density hydrogen and helium in the IGM probed by QSO absorption lines
is sensitive to the amplitude and spectral shape of the metagalactic UV
background. We use realistic HI and HeII Ly-alpha forest spectra, constructed
from state-of-the-art hydrodynamical simulations of a Lambda-CDM Universe, to
confirm the reliability of using line profile fitting techniques to infer the
ratio of the metagalactic HI and HeII ionization rates. We further show that
the large spatial variations and the anti-correlation with HI absorber density
observed in the ratio of the measured HeII to HI column densities can be
explained in a model where the HI ionization rate is dominated by the combined
UV emission from young star forming galaxies and QSOs and the HeII ionization
rate is dominated by emission from QSOs only. In such a model the large
fluctuations in the column density ratio are due to the small number of QSOs
expected to contribute at any given point to the HeII ionization rate. A
significant contribution to UV emission at the HeII photoelectric edge from hot
gas in galaxies and galaxy groups would decrease the expected fluctuations in
the column density ratio. Consequently, this model appears difficult to
reconcile with the large increase in HeII opacity fluctuations towards higher
redshift. Our results further strengthen previous suggestions that observed
HeII Ly-alpha forest spectra at z~2-3.5 probe the tail end of the reionization
of HeII by QSOs.Comment: 14 pages, 7 figures, accepted for publication in MNRAS. Minor changes
to submitted version, most notably an extra section (5.2) and figure (fig.6
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