Is there a dichotomy in the radio loudness distribution of quasars?

Abstract

We present a new approach to tackling the issue of radio loudness in quasars. We constrain a (simple) prescription for the intrinsic distribution of radio-to-optical ratios by comparing properties of Monte Carlo simulated samples with those of observed optically selected quasars. We find strong evidence for a dependence of the radio luminosity on the optical one, even though with a large scatter. The dependence of the fraction of radio-loud quasars on apparent and absolute optical magnitudes results in a selection effect related to the radio and optical limits of current surveys. The intrinsic distribution of the radio-to-optical ratios shows a peak at R 1.4* ∼ 0.3, with only ≲5 per cent of objects being included in a high-R1.4* tail, which identifies the radio-loud regime. No lack or deficit of sources - but only a steep transition region - is present between the radio-loud and radio-quiet populations at any R 1.4*. Briefly, we discuss possible origins for this behaviour (e.g. an absence of jets in radio-quiet sources, a large range of radiative radio efficiency, different lifetimes for the accretion and jet ejection phenomena, etc.).