Orientation effects in quasar spectra: the broad- and narrow-line regions

Abstract

We use the Sloan Digital Sky Survey, along with the NRAO VLA Sky Survey and the Westerbork Northern Sky Survey to define a sample of 746 radio-loud quasars and measure their 330 MHz-1.4 GHz spectral indexes. Following previous authors we take the radio spectral index as an indicator of the orientation towards the quasars such that more pole-on sources tend to have flatter spectral indexes. We use this proxy for the orientation of quasars to investigate the effect orientation may have on optical spectra. Quasars with flatter spectral indexes tend to be brighter. However, we find no indication of reddening in steep-spectrum quasi-stellar objects (QSOs) to indicate obscuration of the accretion disc by a torus as a possible explanation. Nor do we find increased reddening in the flat-spectrum sources which could imply a contribution from jet-related synchrotron emission. We reproduce a previously described anticorrelation between the width of the Mg ii line and radio spectral index that indicates a disc-like geometry for the Mg ii broad-line region (BLR). However, in contrast to previous authors we find no such correlation for the C iv line suggesting a more isotropic high-ionization BLR. Both the [O ii] and [O iii] narrow lines have more flux in steep spectrum sources, while the [O iii]/[O ii] flux ratio is lower in these sources. To describe both of these effects we propose a simple geometric model in which the narrow-line region (NLR) exists primarily on the surface of optically thick clouds facing the active nucleus and the NLR is stratified such that higher ionization lines are found preferentially closer to the nucleus. Quantitatively we find that orientation may effect the observed strength of narrow lines, as well as ratios between lines, by a factor of similar to 2. These findings have implications for the use of [O iii] and [O ii] emission lines to estimate bolometric luminosities, as well as comparisons between narrow-line luminosity functions for type 1 and 2 objects and the potential of emission-line diagnostic diagrams as an accurate tool with which to distinguish types of active galactic nuclei. Finally we find no evidence that broad-absorption line (BAL) QSOs have a different spectral index distribution to non-BALs although we only have 25 obvious BALs in our sample.