APM z ≳ 4 survey: distribution and evolution of high column density HI absorbers

Abstract

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 ≤ z_(absorption) ≤ 4.4 (eight with z_(absorption) > 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 N_(HI) ≥ 10^(20.3) 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 N_(HI) ≥ 17.2. In the Hi column density distribution f (N) = kN^(−β) we find evidence for breaks in the power law, flattening for 17.2 ≤ log N_(HI) ≲ 21 and steepening for log N_(HI) > 21.2. The breaks are more pronounced at higher redshift. The column density distribution function for the data with log N_(HI) ≥ 20.3 is better fitted with the form f (N) = (f_*/N_*) (N/N_*)^(−β)exp (— 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) = N_o(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 N_o = 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.