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| - APM z ≳ 4 survey: distribution and evolution of high column density H i absorbers
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| - Eleven candidate damped Lyα absorption systems were identified in 27 spectra of the quasars from the APM z ≳ 4 survey covering the redshift range 2.8 ≤ zabsorption ≤ 4.4 (eight with zabsorption > 3.5). High-resolution echelle spectra (0.8-Å FWHM) have been obtained for three quasars, including two of the highest redshift objects in the survey. Two damped systems have confirmed H i column densities of NHI ≥ 1020.3 atom cm−2, with a third falling just below this threshold. We have discovered the highest redshift damped Lyα absorber known at z = 4.383 in QSO BR 1202 — 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 < z< 4.7 to study the redshift evolution and the column density distribution function for absorbers with log NHI ≥ 17.2. In the H i column density distribution f (N) = kN−β we find evidence for breaks in the power law, flattening for 17.2 ≤ log NHI ≲ 21 and steepening for log NHI > 21.2. The breaks are more pronounced at higher redshift. The column density distribution function for the data with log NHI ≥ 20.3 is better fitted with the form f (N) = (f*/N*) (N/N*) −βexp ( — N/N*) with log N* = 21.63 ± 0.35, β = 1.48 ± 0.30, and f* = 1.77 × 10−2. We study the evolution of the number density per unit redshift of the damped systems by fitting the sample with the customary power law N(z) = No(1 + z)γ. For a population with no intrinsic evolution in the product of the absorption cross-section and comoving spatial number density this will give γ = 1/2 (Ω = 1) or γ = 1 (Ω = 0). The best maximum-likelihood fit for a single power law is γ = 1.3 ± 0.5 and No = 0.041−0.02+0.03, consistent with no intrinsic evolution even though the value of γ is also consistent with that found for the Lyman limit systems where evolution is detected at a significant level. However, redshift evolution is evident in the higher column density systems with an apparent decline in N(z) for z > 3.5.
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