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
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.
Key words: cosmology: observations / intergalactic medium
© ESO, 2006