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

Abstract

International audienceWe 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 Å). Three main components, spanning about 300 km s-1, can be distinguished in the Lyman limit system at z ~ 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 =~ 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 ~ 1.94 and z ~ 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 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 ~ 1.87, we can estimate the temperature of the gas to be log T <~ 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) gtrsim 14 systems at z <~ 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. Based on material collected during commissioning time of UVES, the UV and Visible Echelle Spectrograph, mounted on the Kueyen ESO telescope operated on Cerro Paranal (Chile) and with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities of Research in Astronomy, Inc. under NASA contract NAS 5-26555