Anisotropic Optical Continuum Emission in Radio Quasars

Summary

Abstract

A preliminary analysis of radio and optical data for the low-frequency-selected Molonglo Quasar Sample provides new evidence that radio beaming, as inferred from core dominance of the radio emission, may be accompanied by an enhancement of the optical continuum.

Introduction

There is growing evidence that the optical continuum of radio quasars is not emitted isotropically (Tadhunter et al. 1987; Penston et al. 1990; Jackson & Browne 1991), implying that magnitude-limited samples could be seriously affected by an orientation bias (Kapahi & Shastri 1987). To minimise this bias we have defined a new complete sample from the 408 MHz Molonglo Reference Catalogue (MRC; Large et al. 1981), based on deep optical identifications from UK Schmidt plates. Quasars selected at low frequency are expected to be dominated by their unbeamed extended radio flux, rather than any relativistically beamed core component, and should therefore have their radio jet axes oriented randomly in the sky.

Sample Definition and Observations

Over 700 MRC radio sources, with peak flux density S₄₀₈ > 0.95 Jy and falling in a 10° declination strip, −20° > δ > −30°, were mapped at 843 MHz in ‘snapshot’ mode with the Molonglo Observatory Synthesis Telescope. Optical counterparts (complete to the plate limit, B_J = 22.5) were then identified from UK Schmidt survey plates. All 82 resulting QSO candidates in two RA regions (09^h–14^h and 20^h–06^h) were subsequently imaged with the VLA at 5 GHz; 31 of these had previously published redshifts.