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Issue A&A
Volume 458, Number 2, November I 2006
Page(s) 427 - 439
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20065372



A&A 458, 427-439 (2006)
DOI: 10.1051/0004-6361:20065372

The evolution of Lyman $\alpha$ absorbers in the redshift range $0.5<\textit{z}<1.9$

E. Janknecht1, D. Reimers1, S. Lopez2 and D. Tytler3

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 $\alpha$ absorbers of the intergalactic medium (IGM) in the largely unexplored redshift range z=0.5-1.9. We use high-resolution ( $R \geq 30\,000$) UV (STIS) and optical (VLT/UVES and Keck/HIRES) spectra of nine bright quasars with $z_{\rm em} < 1.94$. The Ly $\alpha$ 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 $\alpha$ line sample are summarized as follows:

1. The evolution of the number density of the absorbers can be described by the power law $\frac{{\rm d}n}{{\rm d}z} \propto (1+z)^{\gamma}$. The number density of the low column density lines ( $N_{\rm\ion{H}{i}}=(10^{12.90}{-}10^{14.00})$ cm-2) decreases with decreasing z with $\gamma=0.74\pm0.31$ in the interval z=0.7-1.9. A comparison with results at higher redshifts shows that it is decelerated in the explored redshift range and turns into a flat evolution for $z \rightarrow 0$. The stronger absorbers ( $N_{\rm\ion{H}{i}} > 10^{13.64}$ cm-2) thin out faster ( $\gamma=1.50\pm0.45$). The break in their evolution predicted for z=1.5-1.7 cannot be seen down to z=0.7. 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 $N_{\rm HI}=10^{12.90}$ cm-2. It can be approximated by a single power law with the exponent $\beta=1.60\pm0.03$ over almost three orders of magnitude. $\beta$ is redshift independent.

3. The Ly $\alpha$ lines with lower column densities as well as the higher column density lines show marginal clustering with a $2\sigma$ significance over short distances ( $\Delta v < 200$ km s-1 and $\Delta v < 100$ 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 $\overline{b}=(34\pm22)$ 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


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