Correlations of Quasar Optical Spectra with Radio Morphology

TL;DR

This study analyzes a large sample of quasars to explore how their optical spectra correlate with radio morphology and orientation, revealing new insights into the relationship between radio properties and emission line strengths.

Contribution

It provides the first large-scale statistical analysis linking quasar optical spectral features with detailed radio morphology classifications and orientation indicators.

Findings

Radio core emission correlates with redder optical colors.

Optical line EWs increase with core-to-lobe flux ratio (RI).

No strong dependence of EW on radio morphology or Baldwin effect.

Abstract

(Abridged) With the largest homogeneous quasar sample with optical spectra and robust radio morphology classifications assembled to date, we investigate quasar radio and optical properties with unprecedented statistical power. The data consist of 4714 radio quasars from FIRST with S_{20}>2mJy and SDSS spectra. Radio morphology classes include core-only (core), core-lobe (lobe), core-jet (jet), lobe-core-lobe (triple), and double-lobe. We examine the optical colors of radio-morphology subsamples and find that radio quasars with core emission unresolved by FIRST (on 5" scale) have a redder color distribution than radio-quiet quasars (S_{20}<1mJy); other classes of radio quasars have optical color distributions similar to the radio-quiet quasars. This analysis also suggests that optical colors of z<2.7 SDSS quasars are not strongly (<0.1mag) biased blue. We show that the radio core-to-lobe flux density ratio (R) and the radio-to-optical (i-band) ratio of the quasar core (RI) are correlated, suggesting that both parameters are indicative of line-of-sight orientation. We investigate spectral line equivalent widths as a function of R and RI, including the [OIII] narrow line doublet and the CIV {\lambda}1549 and MgII {\lambda}2799 broad lines. We find that the rest equivalent widths (EWs) of the broad lines correlate positively with RI at the 4-8{\sigma} level. But we find no strong dependence of EW on R, in contrast to previous results. One interpretation is that EWs increase as the line-of-sight angle to the radio-jet axis decreases. These results are in stark contrast to commonly accepted orientation-based theories, which suggest that continuum emission should increase as the angle to the radio-jet axis decreases, resulting in smaller EW of emission lines (assumed isotropic). Finally, we find that the Baldwin effect in our sample does not depend on quasar radio morphology.