1996A&A...305..450D

We report the detection of a faint narrow HI absorption line at a redshift of 3.38 against the quasar PKS 0201+113 which itself has an emission redshift of 3.61. The absorption line redshift agrees, to within the errors, with that of a damped Lyα line. The line has a halfwidth of about 9km/s. Comparing the line equivalent width with the HI column density of the neutral gas we deduce a spin temperature for the gas of about 1100K. In view of the fact that the spin temperature is remarkably similar to that derived for other high redshift absorption systems we consider it unlikely that it is due to different lines-of-sight for the ultraviolet continuum and the compact radio source. The high spin temperature may reflect a high kinetic temperature in most of the intervening HI. The HI in high redshift damped Lyα absorbers may therefore contain relatively more warm gas than our own and nearby galaxies. The damped Lyα system in PKS 0201+113 is currently the most distant system detected through the 21-cm line of atomic hydrogen. Its properties appear to be remarkably similar to those of systems at redshifts around 1. If damped Lyα systems are due to protodisks and/or disks of spiral galaxies, then galaxies similar to those of present day spiral galaxies were already forming at a redshift of 3.4. Following Wolfe et al. (1976) we use the agreement between the redshifts of the 21-cm line and the optical resonance lines to constrain the possible variation, over the lookback time to the absorber, of the product of the fine structure constant, nuclear g factor for the proton, and the masses of the electron and proton. We obtain a 3σ upper limit of 5x10^–4 on the change in the ratio α²g_pm_e/m_p at a redshift of z=3.4. The corresponding 3σ upper limit to the rate of change of this ratio is 6x10^–14yr^–1.