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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