The nature of absorbers at high redshift ^*,**

Institut für Physik und Astronomie, Universität Potsdam, Haus 28, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany e-mail: [cfech;prichter]@astro.physik.uni-potsdam.de

Received: 19 June 2008
Accepted: 14 January 2009

Abstract

Aims. We present a study of absorption systems at in the spectra of 19 QSOs, based on data obtained with the VLT/UVES instrument. Our analysis includes both the absorbers arising from the intergalactic medium, as well as systems in the vicinity of the background quasar.

Methods. We construct detailed photoionization models to study the physical conditions and abundances in the absorbers and to constrain the spectral hardness of the ionizing radiation.

Results. The rate of incidence for intervening components is d, corresponding to d. The column density distribution function is fitted by the slope , consistent with measurements of and . The narrow line widths () imply photoionization rather than collisions as the dominating ionization process. The column densities of and are correlated but show different slopes for intervening and associated absorbers, which indicates different ionizing spectra. Associated systems are found to be more metal-rich, denser, and more compact than intervening absorbers. This conclusion is independent of the adopted ionizing radiation. For the intervening systems we find typical values of and and sizes of a few kpc, while for associated absorbers we obtain , and sizes of several 10 pc. The abundance of nitrogen relative to carbon [N/C] and α-elements like oxygen and silicon [ N/α] is correlated with [N/H], indicating the enrichment by secondary nitrogen. The larger scatter in [ N/α] in intervening systems suggests an inhomogeneous enrichment of the IGM. There is an anti-correlation between [ N/α] and [ α/C] , which could be used to constrain the initial mass function of the carbon- and nitrogen-producing stellar population.