The spin temperature of high-redshift damped Lyman α systems

We report results from a programme aimed at investigating the temperature of neutral gas in high-redshift damped Lyman α absorbers (DLAs). This involved (1) H I 21 cm absorption studies of a large sample of DLAs towards radio-loud quasars, (2) very long baseline interferometric studies to measure the low-frequency quasar core fractions, and (3) optical/ultraviolet spectroscopy to determine DLA metallicities and the velocity widths of low-ionization metal lines. Including literature data, our sample consists of 37 DLAs with estimates of the harmonic mean spin temperature Ts. We find a statistically significant (4σ) difference between the Ts distributions in the high-z (z > 2.4) and low-z (z 1.7 have Ts values indicating CNM fractions >20 per cent, comparable to the median value (≈27 per cent) in the Galaxy. We tested whether the H I column density measured towards the optical quasar might be systematically different from that towards the radio core by comparing the H I column densities inferred from H I 21 cm emission studies at different spatial resolutions (≈15 pc-1 kpc) in the Large Magellanic Cloud. The high-resolution NH I values are, on average, larger than the smoothed ones for NH I > 1021 cm-2, but lower than the smoothed NH I estimates for NH I 1. Since all DLAs at z > 1 have angular diameter distances comparable to or larger than those of their background quasars, they have similar efficiency in covering the quasars. We conclude that low covering factors in high-z DLAs cannot account for the observed redshift evolution in spin temperatures.

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