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abstract
Combining samples of CIV and MgII absorption systems obtained from ground-based and HST observations has shown that the evolution of these two classes of absorbers reflects not only an evolution of their average cross-section, but also of their ionization level, opacity to ionizing UV radiation and chemical composition. The dimensions, mass and ionization level of individual absorbing clouds are lower at $z$ = 0.5 than at $z$ = 2, whereas the heavy element abundances are higher, reaching [Z/H] $\simeq -0.5$ at $z$ = 0.5. The latter is higher by a factor 3 to 5 than the values derived for two damped Ly$\alpha$ systems at $z$ = 0.7. This might be explained in terms of different types of absorbing galaxies, at least for those identified so far. Several CIV systems at $z >$ 0.6 have associated OVI absorption. This high ionization phase could be a homogeneus region of large extent, $r \geq$ 50 kpc, and low density, $n_H \leq 3 \times 10^{-4}$ cm$^{-3}$, in which the denser CIV and MgII clouds are embedded.
