Astronomy & Astrophysics

A&A 483, 19-34 (2008)
DOI: 10.1051/0004-6361:20079109

EUV spectral energy distribution of quasars restored from associated absorbers

S. A. Levshakov^{1, 2}, I. I. Agafonova^{3, 2}, D. Reimers¹, J. L. Hou², and P. Molaro⁴

¹ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
e-mail: lev@astro.ioffe.rssi.ru
² Shanghai Astronomical Observatory, 80 Nandan Road, Shanghai 200030, PR China
³ Ioffe Physico-Technical Institute, Polytekhnicheskaya Str. 26, 194021 St. Petersburg, Russia
⁴ Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34131 Trieste, Italy

(Received 20 November 2007 / Accepted 5 February 2008)

Abstract
Aims. We reconstruct the spectral shape of the quasar ionizing radiation in the extreme-UV range (1 Ryd $\leq$ E < 10 Ryd) from the analysis of narrow absorption lines (NAL) of the associated systems.
Methods. The computational technique for inverse spectroscopic problems - a Monte Carlo Inversion augmented by the spectral shape recovering procedure and modified to account for the incomplete coverage of the light source is used to analyze observations.
Results. The ionizing spectra responsible for the ionization structure of the NAL systems require an intensity depression at E > 4 Ryd which is attributed to the $\ion{ii}$ Lyman continuum opacity ( $\tau^{\rm He\,{\scriptscriptstyle II}}_{\rm c} \sim 1$ ). A most likely source of this opacity is a quasar accretion disk wind. The corresponding column density of $\ion$ in the wind is estimated as a few times 10¹⁶ cm^-2. This amount of neutral hydrogen should cause a weak continuum depression at $\lambda \la 912$ Å (rest-frame), and a broad and shallow absorption in $\ion$ Ly- $\alpha$ . If the metallicity of the wind is high enough, other resonance lines of $\ion{O}{vi}$ , $\ion{vi}$ - $\ion{viii}$ , etc. are expected. In the analyzed QSO spectra we do observe broad (stretching over 1000s km s^-1) and shallow ( $\tau \ll 1$ ) absorption troughs of $\ion$ Ly- $\alpha$ and $\ion{O}{vi}$ $\lambda\lambda1031, 1037$ as well as continuum depressions at $\lambda \la 912$ Å which correspond to N( $\ion$ ) ~ 5 $\times$ 10¹⁶ cm^-2. Observational data available in both the UV and X-ray ranges suggest that at least ~50% of the quasar radiation passes through the gas opaque in the $\ion{ii}$ Lyman continuum. This means that the outcoming ionizing spectrum should have a pronounced intensity break at E > 4 Ryd with the depth of this break depending on the angle with the rotational axis of the accretion disk (the larger the angle the deeper the break). The QSO spectra with a discontinuity at 4 Ryd can influence the rate of $\ion{ii}$ reionization in the intergalactic medium and partly explain the inhomogeneous (patchy) ionization structure of the intergalactic $\ion{ii}$ observed at $z\sim3$ .

Key words: cosmology: observations -- line: formation -- line: profiles -- quasars: absorption lines