Inhomogeneous metal enrichment at z ∼ 1.9:: The Lyman limit systems in the spectrum of the HDF-S quasar

We present a detailed analysis of three metal absorption systems observed in the spectrum of the HDF-South quasar J2233-606 (z(em) = 2.238), taking advantage of new VLT-UVES high resolution data (R = 45 000, S/N = 40-60, lambda lambda 3050-10 000 Angstrom). Three main component, spanning about 300 km s(-1), can be distinguished in the Lyman limit system at z similar to 1.92. They show a surprisingly large variation in metallicities, respectively similar to1/500, 1/8 and 1/100 solar. The large value found for the second component at z similar or equal to 1.9259, suggests that the line of sight crosses a star-forming region. In addition, there is a definite correlation between velocity position and ionisation state in this component, which we interpret as a possible signature of an expanding H II region. The systems at z similar to 1.94 and z similar to 1.87 also have high metallicity similar to 1/4 and 1/3 solar. We find that photoionization and collisional ionisation are equal alternatives to explain the high excitation phase revealed by O VI absorption. seen in these two systems. From the width of the Si IV, C IV, Si III and C III lines in the system at z similar to 1.87, we can estimate the temperature of the gas to be log T less than or similar to 4.7, excluding collisional ionisation. Finally, we compute the Si IV/C IV ratio for all Voigt profile components in a sample of log N(C IV) greater than or similar to 14 systems at z less than or similar to 2, The values show a dispersion of more than an order of magnitude and most of them are much larger than what is observed for weaker systems. This is probably an indication that high column density systems preferably originate in galactic halos and are mostly influenced by local ionising sources.