A deep search for 21cm absorption in high redshift damped Lyman-$α$ systems

Nissim Kanekar; Jayaram N Chengalur

doi:10.1051/0004-6361:20021922

paper

A deep search for 21cm absorption in high redshift damped Lyman- $α$ systems

arXiv:astro-ph/0211637 · doi:10.1051/0004-6361:20021922

Abstract

We present deep GMRT 21cm absorption spectra of 10 damped Lyman- $α$ systems (DLAs), of which 8 are at redshifts $z \ga 1.3$. HI absorption was detected in only one DLA, the $z = 0.5318$ absorber toward PKS 1629+12, which has been identified with a luminous spiral galaxy; the spin temperature limit ( $T_{s} \leq 310$ K) derived from our observations continues the trend of DLAs associated with bright spirals having low spin temperatures. In 7 of the remaining 9 systems, the observations place strong lower limits on the spin temperature of the HI gas. The sample of DLAs searched for 21cm absorption now consists of 31 systems, with $T_{s}$ estimates available in 24 cases; of these, 16 are at $z < 2$ and 8 at $z > 2$ , with 11 (all at $z < 1$ ) having optical IDs. For the latter 11 systems, we find that all low $T_{s}$ DLAs have been identified with luminous galaxies, while all high $T_{s}$ ($T_s \ga 1000$ K) DLAs have been identified with either LSBs or dwarfs. DLA spin temperatures thus appear to correlate with galaxy type, with no correlation seen between $T_{s}$ and impact parameter. The trend that low $z$ DLAs exhibit both high and low $T_{s}$ values while high redshift ($z \ga 3$) 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 $z > 2$ and DLAs at $z < 2$ are drawn from the same parent population at ~ 99 % confidence level. Finally, we estimate upper limits on the fraction of cold HI, $f_{C N M}$ , in the $z \ga 3$ DLAs. In local spirals, $f_{C N M} \sim 0.5$ ; in contrast, we find that $f_{C N M} < 0.3$ in all 7 high $z$ DLAs, with $f_{C N M} < 0.1$ in 5 of the 7 cases. (abridged)

13 pages, 5 figures. Accepted for publication in Astronomy & Astrophysics

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The Apparent Host Galaxy of PKS 1413+135: HST, ASCA and VLBA Observations

A deep search for 21cm absorption in high redshift damped Lyman-α systems

Abstract

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A deep search for 21cm absorption in high redshift damped Lyman- $α$ systems