The evolution of Lyman α absorbers in the redshift range

¹ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany e-mail: [ejanknecht;dreimers]@hs.uni-hamburg.de
² Departamento de Astronomia, Universidad de Chile, Casilla 36, Santiago, Chile e-mail: slopez@das.uchile.cl
³ Center for Astrophysics and Space Sciences, University of California, San Diego, MS 0424, La Jolla, CA 92093-0424, USA e-mail: tytler@ucsd.edu

Received: 6 April 2006
Accepted: 28 July 2006

Abstract

We investigate the evolution and the statistical properties of the Ly α  absorbers of the intergalactic medium (IGM) in the largely unexplored redshift range . We use high-resolution () UV (STIS) and optical (VLT/UVES and Keck/HIRES) spectra of nine bright quasars with . The Ly α  lines detected in the lines of sight (LOS) towards these quasars are evaluated with a software package which determines simultaneously the quasar continuum and the line profiles. The main results for the combined Ly α  line sample are summarized as follows:
1. The evolution of the number density of the absorbers can be described by the power law . The number density of the low column density lines ( cm^-2) decreases with decreasing z with in the interval .    A comparison with results at higher redshifts shows that it is decelerated in the explored redshift range and turns into a flat evolution for . The stronger absorbers ( cm^-2) thin out faster (). The break in their evolution predicted for cannot be seen down to . On the other hand, a comparison with values from the literature for the local number density gives a hint that this break occurs at lower redshift.
2. The distribution of the column densities of the absorbers is complete down to  cm^-2. It can be approximated by a single power law with the exponent over almost three orders of magnitude. β is redshift independent.
3. The Ly α  lines with lower column densities as well as the higher column density lines show marginal clustering with a significance over short distances ( km s^-1 and  km s^-1, respectively). We do not see any difference in the clustering with either column density or redshift.
4. The distribution of the Doppler parameters has a mean value of  km s^-1. This value is typical for the analyzed region. It does not change significantly with z.