APM-Z-GREATER-THAN-OR-SIMILAR-TO-4 SURVEY - DISTRIBUTION AND EVOLUTION OF HIGH COLUMN DENSITY H-I ABSORBERS

Eleven candidate damped Ly alpha absorption systems were identified in 27 spectra of the quasars from the APM z greater than or similar to 4 survey covering the redshift range 2.8 less than or equal to z(absorp tion) less than or equal to 4.4 (eight with z(absorption) > 3.5). High -resolution echelle spectra (0.8-Angstrom FWHM) have been obtained for three quasars, including two of the highest redshift objects in the s urvey. Two damped systems have confirmed HI column densities of N-Ht g reater than or equal to 10(20.3) atom cm(-2), with a third falling jus t below this threshold. We have discovered the highest redshift damped Ly alpha absorber known at z = 4.383 in QSO BR 1202 - 0725. The APM Q SOs provide a substantial increase in the redshift path available for damped surveys for z > 3. We combine this high-redshift sample with ot her quasar samples covering the redshift range 0.008 < z < 4.7 to stud y the redshift evolution and the column density distribution function for absorbers with log N-Ht greater than or equal to 17.2. In the HI c olumn density distribution f(N) = kN(-beta) we find evidence for break s in the power law, flattening far 17.2 less than or equal to log N-Ht less than or similar to 21 and steepening for log N-Ht > 21.2. The br eaks are more pronounced at higher redshift. The column density distri bution function for the data with log N-Ht greater than or equal to 20 .3 is better fitted with the form f(N) = (f/N*)(N/N*)(-beta) exp(-N/N ) with log N* = 21.63 +/- 0.35, beta = 1.48 +/- 0.30, and f* = 1.77 x 10(-2). We study the evolution of the number density per unit redshif t of the damped systems by fitting the sample with the customary power law N(z) = N-0(1 + z)(gamma). For a population with no intrinsic evol ution in the product of the absorption cross-section and comoving spat ial number density this will give gamma = 1/2 (Omega = 1) of gamma = 1 (Omega = 0). The best maximum-likelihood fit for a single power law i s gamma = 1.3 +/- 0.5 and N-0 = 0.04(-0.02)(+0.03), consistent with no intrinsic evolution even though the value of gamma is also consistent with that found for the Lyman limit systems where evolution is detect ed at a significant level. However, redshift evolution is evident in t he higher column density systems with an apparent decline in N(z) for z > 3.5.