Key points are not available for this paper at this time.
Eleven candidate damped Lya absorption systems were identified in 27 spectra of the quasars from the APM z>4 survey covering the redshift range 2. 83. 5). High resolution echelle spectra (0. 8A FWHM) have been obtained for three quasars, including 2 of the highest redshift objects in the survey. Two damped systems have confirmed HI column densities of N (HI) >= 10²0. 3 atoms cm^-2, with a third falling just below this threshold. We have discovered the highest redshift damped Lya absorber known at z=4. 383 in QSO BR1202-0725. The APM QSOs provide a substantial increase in the redshift path available for damped surveys for z>3. We combine this high redshift sample with other quasar samples covering the redshift range 0. 008 =17. 2. In the HI column density distribution f (N) =kN^-beta we find evidence for breaks in the power law, flattening for 17. 221. 2. The column density distribution function for the data with log N (HI) >=20. 3 is better fit with the form f (N) = (f*/N*) (N/N*) ^-beta exp (-N/N*). Significant redshift evolution in the number density per unit redshift is evident in the higher column density systems with an apparent decline in N (z) for z>3. 5.
Storrie‐Lombardi et al. (1996) studied this question.