The spin temperature of high-redshift damped Lyman α systems

We report results from a programme aimed at investigating thetemperature of neutral gas in high-redshift damped Lyman αabsorbers (DLAs). This involved (1) H I 21 cm absorption studies of alarge sample of DLAs towards radio-loud quasars, (2) very long baselineinterferometric studies to measure the low-frequency quasar corefractions, and (3) optical/ultraviolet spectroscopy to determine DLAmetallicities and the velocity widths of low-ionization metal lines.Including literature data, our sample consists of 37 DLAs with estimatesof the harmonic mean spin temperature Ts. We find astatistically significant (4σ) difference between theTs distributions in the high-z (z > 2.4) and low-z (z 1.7 have Ts valuesindicating CNM fractions >20 per cent, comparable to the median value(≈27 per cent) in the Galaxy. We tested whether the H I columndensity measured towards the optical quasar might be systematicallydifferent from that towards the radio core by comparing the H I columndensities inferred from H I 21 cm emission studies at different spatialresolutions (≈15 pc-1 kpc) in the Large Magellanic Cloud. Thehigh-resolution NH I values are, on average, larger than thesmoothed ones for NH I > 1021 cm-2,but lower than the smoothed NH I estimates for NHI 1. Since all DLAs at z > 1 haveangular diameter distances comparable to or larger than those of theirbackground quasars, they have similar efficiency in covering thequasars. We conclude that low covering factors in high-z DLAs cannotaccount for the observed redshift evolution in spin temperatures