The z_abs ~ z_em absorption line systems toward QSO J 2233-606 in the Hubble Deep Field South: NE viii lambda lambda 770,780 absorption and partial coverage absorption line systems toward QSO J 2233-606

Abstract

Results of a careful analysis of the highly ionized absorption systems, observed over the redshift range 2.198-2.2215 in the z_em = 2.24 HDFS-QSO J 2233-606, are presented. The strength and covering factor of the O VI and Ne VIII absorption lines suggest that the gas is closely associated with the AGN. In addition, most of the lines show signature of partial coverage and the covering factor varies from species to species. This can be understood if the clouds cover the continuum emission region completely and only a fraction of the broad emission line region. Using photo-ionization models we analyze in more detail the component at z_abs = 2.198, for which we can derive reliable estimates of column densities for H i and other species. Absolute abundances are close to solar but the [N/C] abundance ratio is larger than solar. This result, which is consistent with the analysis of high-z QSO broad emission-lines, confirms the physical association of the absorbing gas with the AGN. The observed column densities of N iv, {N v} and Ne viii favor a two-zone model for the absorbing region where Ne viii is predominantly produced in the highly ionized zone. It is most likely that in QSO J 2233-606, the region producing the Ne viii absorption can not be a warm absorber. One of the Lyalpha absorption lines at z_abs = 2.2215 has a flat bottom typical of saturated lines and non-zero residual intensity in the core, consistent with partial coverage. There is no metal-line from this Lyalpha cloud detectable in the spectrum which suggests either large chemical inhomogeneities in the gas or that the gas is very highly ionized. If the latter is true the cloud could have a total hydrogen column density consistent with that of X-ray absorbers. It is therefore of first importance to check whether or not there is an X-ray warm-absorber in front of this QSO. Based in part on observations obtained with the NASA/ESA Hubble Space Telescope by the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5--26555; observations collected at the European Southern Observatory, La Silla, Chile; and observations collected at the Anglo-Australian Observatory.