The zabs=zem Absorption Line Systems Toward QSO J2233-606 in the Hubble Deep Field South: NeVIII770,780 absorption and partial coverage

Abstract

Results of a careful analysis of the highly ionized absorption systems, observed over the redshift range 2.198--2.2215 in the zem=2.24 HDFS-QSO J2233-606, are presented. Strong OVI and NeVIII absorptions are detected. Most of the lines show signature of partial coverage which 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 zabs = 2.198. 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 NIV, NV and NeVIII favor a two-zone model for the absorbing region where NeVIII is predominantly produced in the highly ionized zone. It is most likely that in QSO J2233-606, the region producing the NeVIII absorption can not be a warm absorber. One of the Lyalpha absorption lines at zabs = 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.