Damped and sub-damped Lyman-? absorbers in z > 4 QSOs

TL;DR

This study surveys damped and sub-damped Lyman-alpha systems at high redshift (z > 2.55) using Keck telescope data, revealing their evolution, distribution, and contribution to neutral gas density in the early universe.

Contribution

It provides the first homogeneous survey of DLAs and sub-DLAs at z > 2.55, analyzing their evolution, distribution, and mass contribution with new observational data.

Findings

Neutral gas density decreases at z > 3.5

H I column density distribution follows a broken power-law

Sub-DLAs contain about 30% of H I mass

Abstract

We present the results of a survey for damped (DLA, log N(H I) > 20.3) and sub-damped Lyman-? systems (19.5 < log N(H I) < 20.3) at z > 2.55 along the lines-of-sight to 77 quasars with emission redshifts in the range 4 < zem < 6.3. Intermediate resolution (R ? 4300) spectra have been obtained with the Echellette Spectrograph and Imager (ESI) mounted on the Keck telescope. A total of 100 systems with log N(H I) > 19.5 are detected of which 40 systems are damped Lyman-? systems for an absorption length of ?X = 378. About half of the lines of sight of this homogeneous survey have never been investigated for DLAs. We study the evolution with redshift of the cosmological density of the neutral gas and find, consis- tently with previous studies at similar resolution, that ?DLA,H I decreases at z > 3.5. The overall cosmological evolution of ?HI shows a peak around this redshift. The H I column density distribution for log N(H I) ? 20.3 is ?tted, consistently with previous surveys, with a single power-law of index ? ? -1.8$\pm$0.25. This power-law overpredicts data at the high-end and a second, much steeper, power-law (or a gamma function) is needed. There is a flattening of the function at lower H I column densities with an index of ? ? ?1.4 for the column density range log N(H I) = 19.5?21. The fraction of H I mass in sub-DLAs is of the order of 30%. The H column density distribution does not evolve strongly from z ? 2.5 to z ? 4.5.