Nature and evolution of Damped Lyman alpha systems

The main properties of Damped Lyman alpha (DLA) systems are briefly reviewed with the aim of studying the nature and evolution of the galaxies associated with this class of QSO absorbers. Candidate DLA galaxies identified at z </= 1 in the fields of background QSOs show a variety of morphological types without a predominance of spirals. Most properties inferred from spectroscopic studies at z >/= 1.65 differ from those expected for spiral galaxies. The observational results instead suggest that a significant fraction of DLA systems originate in low-mass and/or LSB galaxies. Evolution effects are generally not detected in DLA systems. This fact suggests that the differences between the properties of present-day spirals and those of high-z DLA systems may not be ascribed to evolution. Several selection effects can bias the observed population of DLA absorbers. Analysis of these effects indicates that the fraction of spiral galaxies tends to be underestimated relative to the fraction of low-mass or LSB galaxies.