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Issue A&A
Volume 391, Number 2, August IV 2002
Page(s) L11 - L15
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20021003



A&A 391, L11-L15 (2002)
DOI: 10.1051/0004-6361:20021003

Letter

A high-resolution study of the evolution of the Lyman $\alpha$ forest in the redshift interval 0.9 < z < 1.7

E. Janknecht, R. Baade and D. Reimers

Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
    e-mail: [ejanknecht,rbaade,dreimers]@hs.uni-hamburg.de
(Received 15 April 2002 / Accepted 6 July 2002)

Abstract
Spectroscopy with HST/STIS Echelle and VLT/UVES of the bright QSO HE 0515-4414 ( $z_{\rm em} =1.73, B = 15.0$) offers for the first time the opportunity to study the Lyman $\alpha$ forest in the redshift range 0.9 < z < 1.7 at a resolution $\leq$ 10 km s -1. The number density evolution of the Lyman $\alpha$ lines is well described by the power law approach d $n/{\rm d}z \propto (1+z)^\gamma$. We derive $\gamma = 2.23 \pm 1.21$ for the strong lines ( $13.64 \leq {\rm log}\, N_{\rm\ion{H}{i}} \leq 16.00$), in agreement with the Lyman $\alpha$ forest evolution for z > 1.7. The expected slow-down in their evolution does not appear earlier than $z \sim 1$. For the weak lines ( $13.10 \leq {\rm log}\, N_{\rm\ion{H}{i}} \leq 14.00$) we find that the HE 0515-4414 data for z > 1 follow the trend with $\gamma = 1.40$ known from z > 1.7 observations, i.e. we confirm the difference in evolution between weak and strong lines. We use the two-point velocity correlation function (TPCF) to search for clustering of the Lyman $\alpha$ lines, yet we detect no excess in the TPCF on scales up to 10 000 km s -1.


Key words: cosmology: observations -- intergalactic medium -- quasars: Ly $\alpha$ forest -- quasars: individual: HE 0515-4414

Offprint request: E. Janknecht, ejanknecht@hs.uni-hamburg.de


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