164 research outputs found
Direct Observational Test Rules Out Small MgII Absorbers
Recent observations suggest the incidence of strong intervening MgII
absorption systems along the line-of-sight to gamma ray burst (GRB) afterglows
is significantly higher than expected from analogous quasar sightlines. One
possible explanation is a geometric effect, arising because MgII absorbers only
partially cover the quasar continuum regions, in which case MgII absorbers must
be considerably smaller than previous estimates. We investigate the production
of abnormal absorption profiles by partial coverage and conclude that the lack
of any known anomalous profiles in observed systems, whilst constraining,
cannot on its own rule out patchy MgII absorbers.
In a separate test, we look for differences in the distribution function of
MgII equivalent widths over quasar continuum regions and CIII] emission lines.
We show that these anomalies should be observable in any scenario where MgII
absorbers are very small, but they are not present in the data. We conclude
that models invoking small MgII cloudlets to explain the excess of absorbers
seen towards GRBs are ruled out.Comment: Accepted for publication in MNRAS Letters. 5 pages, 2 figure
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
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