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

¹ Instituto de Astronomía, UNAM, Ap. 70-264, 04510 DF, México e-mail: binette@astroscu.unam.mx
² Mt. Stromlo Observatory, Cotter Rd., Weston Creek, ACT 2611, USA e-mail: bgroves@mso.anu.edu.au
³ Dept. of Natural Sciences, Univ. of Hertfordshire, College Lane, Hatfield, Herts AL10 9AB, UK
⁴ ST-ECF, Karl Schwarzschild Str. 2, 85748 Garching-bei-München, Germany e-mail: rfosbury@eso.org
⁵ Dept. of Physics, Rochester Institute of Technology, 85 Lomb Memorial Dr., Rochester, NY 14623-5603, USA

Corresponding author: B; Groves, bgroves@mso.anu.edu.au

Received: 19 November 2002
Accepted: 8 May 2003

Abstract

The gravitational arc observed by Holden et al. at reveals strong emission lines of , ,  but no . 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 () photoionized by such filtered continuum with those produced by a zero-metallicity star of  K. We find that the latter generally produces stronger emission lines of C, N and O than the absorbed powerlaw, even when their respective /Hβ ratio is matched. The absorbed powerlaw generates ,  and  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  and  lines and the  and  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.