A robust derivation of the tight relationship of radio core dominance to inclination angle in high redshift 3CRR sources

TL;DR

This paper establishes a robust semi-empirical relationship between radio core dominance and the inclination angle of high-redshift 3CRR radio sources, enabling accurate orientation inference with minimal dispersion.

Contribution

It provides the first reliable conversion recipe linking radio core dominance to inclination angle, validated against optical and infrared indicators, for high-redshift radio-loud AGN.

Findings

Radio core dominance R correlates strongly with optical type and infrared ratios.

Estimated torus half-opening angles are close to 60 degrees, consistent with large surveys.

Inclination angles can be inferred with approximately 10 degrees accuracy using R.

Abstract

It is believed that, in radio-loud active galactic nuclei (AGN), the core radio flux density can be normalized to the flux density of the extended lobe emission to infer the orientation of a radio source. However very little is known about the reliability and precision of this method, and we are unaware of any robust conversion recipe to infer the inclination from the core dominance. Investigating whether or not the radio core dominance parameter R separates the quasars from the radio-galaxies in the $z \ge$~1 3CRR catalog, we found excellent agreement of R with optical type, infrared flux ratios and optical polarization. This indicates that probably both R and optical classification are very good orientation indicators, and the unified model is strongly predictive for these objects. The relative number densities indicate half-opening angles close to 60$^\circ$, as expected from large surveys. The separations of optical types according to radio core dominance as well as NIR/MIR ratios, which are essentially perfect, means that there can be only a small dispersion of torus half-opening angles. Also, even though torus dust is thought to be clumpy, there is an almost zero probability to see a type-1 source at high inclination. Finally, using only the Copernican Principle, i,e, the assumption that solid angle is filled uniformly with source axis orientations, we estimated a semi-empirical relation between core dominance and AGN inclination. This makes it possible to use R to infer the inclination of a source to an accuracy of $\sim$ 10 degrees or less, at least for this type of object.