High-z nebulae:: Ionization by stars or by an obscured QSO?

The gravitational arc observed by Holden et al. at z = 3.356 reveals strong emission lines of N i v] lambda1485, C i v lambdalambda1549, O III] lambda1665 but no N v lambda1240. The lines were modelled assuming photoionization by a thermal distribution hotter than 60 000 K. This paper considers an alternative ionizing source consisting of a partially absorbed powerlaw. We compare the line ratios from a low metallicity nebula (Z(total) = 0.05Z(.)) photoionized by such filtered continuum with those produced by a zero-metallicity star of T-eff = 80 000 K. We find that the latter generally produces stronger emission lines of C, N and O than the absorbed powerlaw, even when their respective He II / Hbeta ratio is matched. The absorbed powerlaw generates N v lambda1240, O v i lambda1035 and [Ne v] lambda3426 nebular lines weaker than the canonical direct powerlaw, although at a much higher level than the 80 000 K stellar atmosphere. As a result of the large partially ionized zone created by the absorbed powerlaw, the optical [O I] lambda6300 and [S II] lambda6731 lines and the C II] lambda2326 and Mg II lambdalambda2800 lines in the UV are much stronger than in the stellar case. These constitute the best discriminant for differentiating the predictions of the absorbed case from those of the stellar model.