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Astron. Astrophys. 353, 457-464 (2000)

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1. Introduction

The amount of information about the galaxy population at high redshift ([FORMULA]) has increased tremendously in the last few years. Using the Lyman-break technique several hundred high redshift star forming galaxies, Lyman-break galaxies (LBGs), have been detected and studied with imaging and spectroscopy (Steidel et al. 1996). It is, however, not yet known how complete the Lyman-break technique is in detecting high redshift galaxies.

An independent route along which to study the galaxy population at high redshift is via the high column density QSO absorption lines systems. The advantage of high column density QSO absorption lines systems is that a wealth of information on the chemical evolution can be and has been obtained by studying the metallicity and dust content of the absorbers from high resolution spectroscopic studies of the background QSOs (e.g. Lu et al. 1996). However, the spectroscopic studies will not tell us anything about the emission properties of the absorbing galaxies.

Only when combining the information obtained from the LBG-studies (e.g. the luminosity function of LBGs), the absorption line statistics for QSO absorption lines systems and the properties of galaxy counterparts of QSO absorption lines systems can we hope to disentangle the observational selection biases which each of the different studies suffer from and obtain a more complete insight into the nature of the high redshift galaxy population.

The absorption line systems with the highest HI column density, [FORMULA] cm-2, are the Damped Ly[FORMULA] Absorbers (DLAs, Wolfe et al. 19861. Such high HI column densities are at low redshift only found in the disks of spiral galaxies. It it also interesting to note, that active star formation in spiral galaxies only occurs when the HI column density of the disk exceeds [FORMULA] (Kennicutt 1989). For these reasons, DLAs are widely believed to trace the central parts of forming galaxies. Much telescope time has been dedicated to the narrow band imaging of DLAs over the past decade (e.g. Lowenthal et al. 1995 and references therein), but so far resulting in only two confirmed detections for the DLAs towards PKS0528-250 (Moller & Warren 1993a,b; Warren & Moller 1996), and Q0151+048A (Moller et al. 1998; Fynbo et al. 1999a) In addition a spectroscopically confirmed broad band detection of the DLA towards DMS2247-0209 (Djorgovski 1998) and a purely spectroscopic detection of Ly[FORMULA] emission from the DLA towards Q2059-360 have been reported (Pettini et al. 1995, Leibundgut & Robertson 1998), as well as a number of DLA candidates, which have not yet been confirmed by spectroscopy (Steidel et al. 1994, 1995, Aragón-Salamanca et al. 1996, LeBrun et al. 1997).

QSO absorption line systems with N(HI) larger than a few times [FORMULA] cm-2 are optically thick at the Lyman limit and refered to as Lyman Limit Systems (LLSs). Lyman limit absorption is believed to occur in extended gaseous haloes of galaxies, because the neutral hydrogen column density is much larger than in the intergalactic medium and the gas is not predominantly neutral as in DLAs. At low redshifts, where the Lyman limit cannot be observed from the ground, LLS are thought to be traced by MgII absorption, because MgII absorption only occurs in optically thick clouds (Schaeffer 1983). The extensive study of the galaxy counterparts of MgII absorbers presented in Guillemin & Bergeron (1997) shows that MgII absorption predominantly occurs in Sbc or Scd galaxies, but that the objects range from ellipticals to irregular galaxies. At high redshifts the Lyman Break technique were originally used to look for galaxies responsible for Lyman limit absorption in QSO spectra (Steidel & Hamilton 1992). Two candidates were found in six fields (Steidel et al. 1995), but for only one of these, the LLS towards Q2233+131, has confirming spectroscopy been published (Djorgovski et al. 1996).

Three out of the four confirmed high redshift DLAs that have been detected in emission at present are at approximately the same redshift as the background QSO. In order to examine whether [FORMULA] systems indeed are more active emitters (e.g. due to induced star formation or to photoionisation by the QSO; for the suggested mechanisms see Moller & Warren 1993b and Fynbo et al. 1998) we have initiated a programme aimed at studying the galaxy counterparts of this special subgroup of high column density QSO absorption line systems. As part of this programme we chose to study the quasar Q1205-30 of which a spectrum published by Lanzetta et al. (1991) shows the presence of a strong LLS close to the emission redshift of the QSO. The redshift of the quasar is [FORMULA].

In Sect. 2 of this paper we describe the observations obtained with the ESO New Technology Telescope (NTT) and the basic data reduction. In Sect. 3 we discuss the photometry, the selection of Ly[FORMULA] emission line candidates, and the correction for the quasar point spread function. In Sect. 4 we discuss our results. We adopt a Hubble constant of 100h-1 km s-1 Mpc-1 and assume [FORMULA]=1 and [FORMULA]=0 unless otherwise stated.

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Online publication: December 17, 1999