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 ($z_{\rm em}=2.238$), taking advantage of new VLT-UVES high resolution data ($R=45 000$, $S/N=40{-}60$, $\lambda\lambda 3050{-}10 000$ A). Three main components, spanning about 300 km s -1 , can be distinguished in the Lyman limit system at $z\sim 1.92$. They show a surprisingly large variation in metallicities, respectively ~1/500, $1/8$ and $1/100$ solar. The large value found for the second component at $z\simeq1.9259$, 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 Hii region. The systems at $z\sim 1.94$ and $z\sim 1.87$ also have high metallicity, ~1/4 and $1/3$ solar. We find that photoionisation and collisional ionisation are equal alternatives to explain the high excitation phase revealed by Ovi absorption, seen in these two systems. From the width of the Siiv, Civ, Siiii and Ciii lines in the system at $z\sim 1.87$, we can estimate the temperature of the gas to be $\log T \lesssim4.7$, excluding collisional ionisation. Finally, we compute the Siiv/Civ ratio for all Voigt profile components in a sample of $\log N$(Civ)$ \gtrsim 14$ systems at $z \lesssim2$. 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.