The AGN Component in Deep Radio Fields: Current Understanding

TL;DR

This paper reviews the current understanding of the AGN component in deep radio fields, highlighting the dominance of radio-loud AGNs at higher flux densities and the emergence of radio-quiet AGNs at lower flux densities, using multi-frequency data.

Contribution

It provides a comprehensive analysis of the AGN populations in deep radio surveys, emphasizing the potential to identify radio-quiet AGNs through IR color diagnostics.

Findings

Radio-loud AGNs dominate down to ~400 microJy flux densities.

Radio-quiet AGNs become significant below 100 microJy.

IR colors are effective in distinguishing AGN-triggered sources from star-forming ones.

Abstract

The present paper reviews our current understanding of the AGN component in sub-mJy radio fields, as it results from the exploitation of multi-frequency information available in two deep extra-galactic radio fields: the ATESP 5 GHz sample and the First Look Survey. One of the key issues addressed here is whether low-power AGNs are more related to efficiently accreting systems (mostly radio-quiet) or to systems with very low accretion rates (mostly radio-loud). The emerging picture is the following. Radio-loud jet-dominated radio galaxies seem to be largely dominant down to flux densities of the order of e.g. S>400 microJy. At lower flux densities (S(1.4 GHz) > 100 microJy) radio-loud AGN are still present in significant numbers. However a population of radio-emitting AGNs, whose properties are consistent with those expected from existing radio-quiet AGN modeling, clearly shows up. This may indicate that the bulk of the radio-quiet AGN population could emerge from studies of deeper (S<100 microJy) radio samples. The radio-quiet AGN component could be recognised thanks to the availability of IR colors which prove to be especially useful to efficiently separate radio sources triggered by AGNs, from sources triggered by star-formation.