Astron. Astrophys. 333, 841-863 (1998)

1. Introduction

The study of high redshift damped Ly absorption systems (hereafter DLAS) detected in the optical spectra of QSOs is now recognized as a powerful means to investigate the properties of distant galaxies or protogalaxies. Many characteristics of the associated intervening objects can be determined from absorption line data: H I column density, metallicity, ionization state and velocity distribution of the gas. For radio-loud quasars, even more physical parameters are accessible such as the temperature of the H I gas (if 21 cm absorption has been searched for) or Faraday rotation measures induced by the intervening plasma. The large values together with the low ionization degree and small velocity dispersion inferred from ground-based observations for the absorbers bear much resemblance to the properties observed in local galactic disks (Wolfe et al. 1986). Further, in average, metals are observed to be underabundant with respect to Solar values but by a factor of typically 10 (Pettini et al. 1997a) which is naturally accounted for by cosmic evolution (Pei & Fall 1995). DLAS are therefore believed to trace the internal regions of distant "normal" galaxies or their progenitors up to the redshifts of the most distant QSOs known to date ().

To verify this assumption in a direct way, and especially investigate the contribution of faint objects (such as dwarf or low surface-brightness galaxies) that QSO absorption line studies have the power to reveal, we have undertaken a HST study of damped (or candidate damped) Ly systems at low z. Indeed, because of their anticipated faintness and low angular separation to the QSO, it is very difficult to detect and characterize the associated intervening objects using images obtained from the ground (see e.g. Boissé & Boulade 1990; Steidel et al. 1994a). In the latter, the presence of normal galaxies can be detected at low impact parameters but their morphological type, magnitude or extent can hardly be determined due to blending with the bright QSO image. Fainter objects or galaxies at angular separations smaller than about cannot be reliably detected.

In a previous paper (Le Brun et al. 1997, hereafter Paper I), we used high angular resolution HST images to search for galaxies close to the line of sight of QSOs with damped (or candidate damped) Ly systems. Thanks to a careful subtraction of the QSO image, we could show that galaxy-like objects are always present at small impact parameters and that, contrary to Mg II absorbers, damped Ly lines are associated with objects displaying a broad range of morphologies and surface brightness.

The present paper is devoted to a spectroscopic study of the same QSO sample. Our primary aim is to complement data already obtained on a few intermediate redshift systems (Steidel et al. 1993; Cohen et al. 1994; Cohen et al. 1996) and investigate several other cases that have not yet been studied. More specifically we wish i) to determine the H I column density and confirm the damped nature of each system, ii) to detect metal lines suitable for measuring the relative abundance of heavy elements and iii) to investigate other properties such as the amount of dust and molecules. Since we now have some information on the morphological type, luminosity and extent of the intervening galaxies and know which part of them is probed by the QSO sightline, QSO spectra can give important clues to connect the absorption line data to our knowledge of nearby galaxies. More generally, such a global study of systems at intermediate redshift (i.e. intermediate look-back time) is essential to better understand the cosmic evolution of the damped Ly absorbers from up to (Pettini et al. 1997a).

This paper is organized as follows: in Sect. 2, we present the observations and data analysis. The main new results emerging from the spectra are summarized for each target in Sect. 3 while in Sect. 4, we discuss the properties of the DLAS that can be inferred from the new FOS data. Some other metal-rich systems of interest are presented in Sect. 5. Finally, we discuss the implications of these observations on the cosmic evolution of damped Ly absorbers and the bias induced by dust extinction on the apparent properties of the large systems.

© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998

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