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| The evolution of metals (top) and molecular
hydrogen abundance (bottom) in the early Universe. The filled
circles represent the DLAs which exhibit H2 absorption (least-squares
fitted with long dashes) and the small unfilled diamonds represent
the general DLA population (short dashes). |
Although the exact nature of Damped Lyman-alpha absorption systems
(DLAs) is open to debate, i.e. whether they are the result of galactic
disks or small dim galaxies, these dense clouds of gas which intersect
the lines-of-sight to distant quasars provide excellent probes of
the early Universe. As well as independently testing the big bang
model, studies of the primordial and processed gas in these systems
can be used to determine the evolution of galaxies and their chemical
abundances.
DLAs are identified through their absorption of the Lyman-alpha
transition of neutral hydrogen of column densities in excess of
>1020 atoms cm-2. Although, through metal
(elements heavier than helium) absorption features many other atomic
species are known to be present, DLAs are very poor in molecular
content. This is evident through optical studies of molecular hydrogen
and the tracer molecule carbon monoxide. These studies are restricted
to redshifts of z >1.8 (>109 light years distant),
where the Lyman and Werner H2 ultra-violet bands, normally
blocked by the Earth’s atmosphere, are sufficiently redshifted
into the optical band. Low molecular fractions have also been confirmed
at low redshift by our extensive observations of the rotational
transitions of molecular tracers, which are observable with ground-based
millimetre wave telescopes.
However, although the molecular abundances are generally much lower
than in our own Galaxy, molecular hydrogen has been detected in
8 DLAs (approximately 1/5 of those searched). If we plot this abundance
against redshift (see figure) we find a surprisingly tight anti-correlation.
A redshift - metallicity correlation is also apparent in which the
evolution appears to be steeper and tighter than that of the general
population. This suggests that the H2-bearing DLAs identify
a narrower class of object, which could prove very useful in providing
a probe, free of the general DLA population biases, of the chemical
evolution of the Universe.
Steve Curran and John Webb
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