EDP Sciences
Free access
Volume 383, Number 1, February III 2002
Page(s) 91 - 97
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20011738

A&A 383, 91-97 (2002)
DOI: 10.1051/0004-6361:20011738

The CORALS survey

II. Clues to galaxy clustering around QSOs from $z_\mathsf{abs} \sim z_\mathsf{em}$ damped Lyman alpha systems
S. L. Ellison1, L. Yan2, I. M. Hook3, M. Pettini4, J. V. Wall5 and P. Shaver6

1  European Southern Observatory, Casilla 19001, Santiago 19, Chile
2  SIRTF Science Center, Caltech, California, USA
    e-mail: lyan@ipac.caltech.edu
3  Astrophysics: Department of Physics, Nuclear and Astrophysics Laboratory, Keble Road, Oxford OX1 3RH, UK
    e-mail: ihook@gemini.edu
4  Institute of Astronomy, Madingley Rd., Cambridge CB3 0HA, UK
    e-mail: pettini@ast.cam.ac.uk
5  Astrophysics: Department of Physics, Nuclear and Astrophysics Laboratory, Keble Road, Oxford OX1 3RH, UK
    e-mail: jvw@astro.ox.ac.uk
6  European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei Munchen, Germany
    e-mail: pshaver@eso.org

(Received 14 August 2001 / Accepted 4 December 2001 )

We present a list of $z_{\rm abs} \sim z_{\rm em}$ ( $\Delta v < 3000$ km s -1) DLAs discovered during the CORALS survey for absorbers in a radio-selected QSO sample. On the assumption that $z_{\rm abs} \sim z_{\rm em}$ DLAs are neither ejecta from the QSO, nor part of the host galaxy itself, we use the number density statistics of these DLAs to investigate galaxy clustering near the QSO redshift. We find that n(z) of $z_{\rm abs} \sim z_{\rm em}$ DLAs in our radio-selected QSO sample is $\sim $4 times larger than the number density of intervening DLAs, implying an excess of galaxies near the QSO. This result is further supported with the inclusion of the radio-loud QSOs in the FIRST survey, although the total number of DLAs is still small (4) and the result is currently only significant at approximately the $2\sigma$ level. Since all of the $z_{\rm abs} \sim z_{\rm em}$ DLAs we identify in CORALS are found towards optically bright ( B < 20) QSOs, there is no strong evidence (based on these limited statistics) that this population suffers from a severe dust bias. We compare our results with those from an optically-selected, radio-quiet QSO sample in order to determine whether there is evidence for an excess of galaxies around radio-loud versus radio-quiet QSOs. We find that the n(z) of $z_{\rm abs} \sim z_{\rm em}$ DLAs towards radio-quiet QSOs is in agreement with the number density of intervening absorbers. This result, although currently limited by the small number statistics of our survey, supports the conclusion that radio-loud quasars are found preferentially in rich galaxy environments at high redshift. Finally, we report that one of the new $z_{\rm abs} \sim z_{\rm em}$ DLAs discovered by CORALS has some residual flux in the base of the Lyman  $\alpha$ trough which may be due to Lyman  $\alpha$ emission, either from star formation in the DLA galaxy or from gas photoionised by the QSO.

Key words: quasars: general -- quasars: absorption lines -- galaxies: evolution -- galaxies: clusters: general

Offprint request: S. Ellison, sellison@eso.org

SIMBAD Objects

© ESO 2002