The nature of compact radio sources: the case of FR0 radio galaxies

TL;DR

This paper reviews the current understanding of FR0 radio galaxies, a prevalent class of compact, low-luminosity radio sources, highlighting their properties, differences from other FR classes, and future research directions with next-generation telescopes.

Contribution

It provides a comprehensive review of FR0 radio galaxies, discussing their characteristics, differences from FRIs and FRIIs, and the open questions for future radio telescope studies.

Findings

FR0s are the most common radio galaxy class locally.

FR0s have higher core dominance and smaller jets than FRIs.

FR0s share host and nuclear properties with FRIs.

Abstract

Radio-loud compact radio sources (CRSs) are characterised by morphological compactness of the jet structure centred on the active nucleus of the galaxy. Most of the local elliptical galaxies are found to host a CRS with nuclear luminosities lower than those of typical quasars, $\lesssim$10$^{42}\, {\rm erg\, s}^{-1}$. Recently, low-luminosity CRSs with a LINER-like optical spectrum have been named Fanaroff-Riley (FR) type 0 to highlight their lack of substantially extended radio emission at kpc scales, in contrast with the other Fanaroff-Riley classes, full-fledged FRIs and FRII radio galaxies. FR0s are the most abundant class of radio galaxies in the local Universe, and characterised by a higher core dominance, poorer Mpc-scale environment and smaller (sub-kpc scale, if resolved) jets than FRIs. However, FR0s share similar host and nuclear properties with FRIs. A different accretion-ejection paradigm from that in place in FRIs is invoked to account for the parsec-scale FR0 jets. This review revises the state-of-the-art knowledge about FR0s, their nature, and which open issues the next generation of radio telescopes can solve in this context.