than spirals as the progenitors of at least some of the DLA systems.'
author:
- 'F. Matteucci , P. Molaro , G. Vladilo'
date: 'Received date; accepted date'
title: 'Chemical evolution of Damped Ly$\alpha$ systems'
---

Introduction
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Absorption line systems detected in the spectra of Quasi Stellar Objects (QSO) originate in intervening galaxies or protogalaxies. Among the different classes of absorbers, the damped Lyman $\alpha$ systems (DLA) are those characterized by N(HI) $\ge$ 10$^{20}$ cm$^{-2}$ and by showing many low ionization species such as FeII, SiII, CrII, ZnII and OI, which are in their dominant ionization stage for a HI region. The high accuracy in the hydrogen column density determination derived from the damped wings and the absence of significant ionization corrections allow accurate absolute abundance determinations of the gas phase elements. Since DLA systems are observable up to the highest redshift they provide a unique tool for the study of the cosmic chemical history of the early universe. Such observations are complementary to the Hubble Deep Field ones (Mobasher et al. 1996) which show that high redshift galaxies have a large variety of morphological types including many peculiar objects with traces of interaction and merging: interestingly, a large 