Radio and spectroscopic properties of miniature radio galaxies: revealing the bulk of the radio-loud AGN population

TL;DR

This study investigates miniature radio galaxies, revealing they extend known relationships to lower luminosities but lack extended radio emission, indicating most radio-loud AGN are faint and underexplored.

Contribution

It demonstrates that the majority of radio-loud AGN are low-power, compact sources that are underrepresented in flux-limited samples, challenging previous focus on extended radio structures.

Findings

Miniature radio galaxies follow extended relationships at low luminosity.

They lack significant extended radio emission compared to classical FRI galaxies.

Most radio-loud AGN are compact and faint, forming the bulk of the population.

Abstract

We explore radio and spectroscopic properties of a sample of 14 miniature radio galaxies, i.e. early-type core galaxies hosting radio-loud AGN of extremely low radio power, 10^(27-29) erg s^(-1) Hz^(-1) at 1.4 GHz. Miniature radio galaxies smoothly extend the relationships found for the more powerful FRI radio galaxies between emission line, optical and radio nuclear luminosities to lower levels. However, they have a deficit of a factor of ~100 in extended radio emission with respect to that of the classical example of 3CR/FRI. This is not due to their low luminosity, since we found radio galaxies of higher radio core power, similar to those of 3CR/FRI, showing the same behavior, i.e. lacking significant extended radio emission. Such sources form the bulk of the population of radio-loud AGN in the Sloan Digital Sky Survey. At a given level of nuclear emission, one can find radio sources with an extremely wide range, a factor of >~100, of radio power. We argue that the prevalence of sources with luminous extended radio structures in flux limited samples is due to a selection bias, since the inclusion of such objects is highly favored. The most studied catalogues of radio galaxies are thus composed by the minority of radio-loud AGN that meet the physical conditions required to form extended radio sources, while the bulk of the population is virtually unexplored.