The new class of FR0 radio galaxies

TL;DR

This paper introduces FR0 radio galaxies, a dominant local population of compact, core-dominated radio-loud AGN lacking extended emission, distinct from traditional FRI/FRII classes, with implications for galaxy evolution.

Contribution

It identifies and characterizes FR0 radio galaxies as a new, prevalent class of compact radio-loud AGN, expanding understanding of radio galaxy diversity.

Findings

FR0 galaxies are core-dominated with large black holes.

They lack extended radio emission unlike FRI/FRII.

FR0s are the most common radio class locally.

Abstract

Are the FRI and FRII radio galaxies representative of the radio-loud (RL) AGN population in the local Universe? Recent studies on the local low-luminosity radio sources cast lights on an emerging population of compact radio galaxies which lack extended radio emission. In a pilot JVLA project, we study the high-resolution images of a small but representative sample of this population. The radio maps reveal compact unresolved or slightly resolved radio structures on a scale of 1-3 kpc. We find that these RL AGN live in red massive early-type galaxies, with large black hole masses ($\gtrsim$10$^{8}$ M$_{\odot}$), and spectroscopically classified as Low Excitation Galaxies, all characteristics typical of FRI radio galaxies which they also share the same nuclear luminosity with. However, they are more core dominated (by a factor of $\sim$30) than FRIs and show a clear deficit of extended radio emission. We call these sources 'FR0' to emphasize their lack of prominent extended radio emission. A posteriori, other compact radio sources found in the literature fulfill the requirements for a FR0 classification. Hence, the emerging FR0 population appears to be the dominant radio class of the local Universe. Considering their properties we speculate on their possible origins and the possible cosmological scenarios they imply.