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

摘要

We present the results of a survey of damped (DLA, H I)>20.3)and sub-damped Lyman-systems (H I)<20.3)at z>2.55 along the lines-of-sight to 77quasars with emission redshifts in the range .Intermediate resolution ()spectra were obtained with the Echellette Spectrograph and Imager (ESI)mounted on the Keck telescope. A total of 100 systems with H I)>19.5were detected of which 40 systemsare damped Lyman-systems for an absorption length of .About half of the lines of sight of this homogeneous survey have neverbeen investigated for DLAs.We study the evolution with redshift of the cosmological density of theneutral gas andfind, consistent with previous studies at similar resolution, that decreases at z>3.5. The overall cosmologicalevolution of shows a peak around this redshift. The H Icolumndensity distribution for HIis fitted, consistent with previous surveys, with a single power-law ofindex .This power-law overpredicts data at the high-endand a second, much steeper, power-law (or a gamma function) is needed.There is a flattening of the function at lower H Icolumndensities with an index of for the column density range H I)=19.5-21. The fraction ofH Imass in sub-DLAs isof the order of 30%. The H Icolumndensity distribution does not evolve strongly from to .Key words: Galaxy: evolution - Galaxy:formation - quasars: absorption lines - intergalactic medium -cosmology: observations