Issue |
A&A
Volume 399, Number 3, March I 2003
|
|
---|---|---|
Page(s) | 857 - 868 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20021922 | |
Published online | 14 February 2003 |
A deep search for 21-cm absorption in high redshift damped Lyman-α systems
1
Kapteyn Institute, University of Groningen, Post Bag 800, 9700 AV Groningen, The Netherlands
2
National Centre for Radio Astrophysics, Post Bag 3, Ganeshkhind, Pune 411 007, India e-mail: chengalu@ncra.tifr.res.in
Corresponding author: Nissim Kanekar, nissim@astro.rug.nl
Received:
18
September
2002
Accepted:
20
November
2002
We present deep GMRT 21-cm absorption spectra of 10 damped Lyman-α
systems (DLAs), of which 8 are at redshifts . HI absorption was detected
in only one DLA, the
absorber toward PKS 1629+12. This absorber has been
identified with a luminous spiral galaxy; the spin temperature limit (
K)
derived from our observations continues the trend of DLAs associated with bright spirals
having low spin temperatures. In seven of the remaining 9 systems, the observations place
strong lower limits on the spin temperature of the HI gas.
We combine this sample with data taken from the literature to study the
properties of all known DLAs with 21-cm absorption studies. The sample of DLAs which have
been searched for 21-cm absorption now consists of 31 systems, with Ts estimates
available in 24 cases; of these, 16 are at
and 8 at
, with 11 (all at
)
having optical identifications. For the latter 11 DLAs, we find that all of the
low Ts DLAs have been identified with large, luminous galaxies, while all the DLAs
with high spin temperature (
K) have been identified either with LSBs
or dwarfs. Further, we find no correlation between impact parameter and spin
temperature; it is thus unlikely that the high measured Ts values for DLAs
arise from lines of sight passing through the outskirts of large disk galaxies.
Instead, the spin temperature of DLAs appears to correlate with the host galaxy type.
The trend (noted earlier by Chengalur & Kanekar 2000) that low z DLAs
exhibit both high and low Ts values while high redshift (
) DLAs only show high
spin temperatures is present in this expanded data set. Based on this difference in
spin temperatures, the Gehan test rules out the hypothesis that DLAs at
and
DLAs at
are drawn from the same parent population at ∼99% confidence
level.
Finally, we use the new GMRT spectra along with 2 spectra from the literature
to estimate upper limits on the fraction of cold HI, fCNM, in DLAs at
.
For local spirals,
; in contrast, we find that
in all 7 high z absorbers, and
in 5 of the 7 cases.
Key words: galaxies: evolution: / galaxies: formation: / galaxies: ISM / cosmology: observations / radio lines: galaxies
© ESO, 2003
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