COLA III. Radio Detection of AGN in Compact Moderate Luminosity Infra-Red Galaxies

TL;DR

This study uses high-resolution radio observations to identify active galactic nuclei in moderate luminosity infrared galaxies, revealing a strong link between nuclear starbursts and AGN activity.

Contribution

It demonstrates that VLBI-detected radio cores in star-forming galaxies are predominantly powered by AGN, highlighting a close connection between starburst environments and AGN activity.

Findings

High fraction (20/90) of VLBI sources indicate prevalent AGN activity.

VLBI core luminosities exceed supernova remnant levels, confirming AGN origin.

Detected AGN are associated with compact nuclear starburst regions.

Abstract

We present results from 4.8 GHz VLA and Global-VLBI observations of the northern half of the moderate FIR luminosity (median L_IR = 10^11.01 L_Sol) COLA sample of star-forming galaxies. VLBI sources are detected in a high fraction (20/90) of the galaxies observed. The radio luminosities of these cores (~10^21 W/Hz) are too large to be explained by radio supernovae or supernova remnants and we argue that they are instead powered by AGN. These sub-parsec scale radio cores are preferentially detected toward galaxies whose VLA maps show bright 100-500 parsec scale nuclear radio components. Since these latter structures tightly follow the FIR to radio-continuum correlation for star-formation we conclude that the AGN powered VLBI sources are associated with compact nuclear starburst environments. The implications for possible starburst-AGN connections are discussed. The detected VLBI sources have a relatively narrow range of radio luminosity consistent with models in which intense compact Eddington-limited starbursts regulate the gas supply onto a central super-massive black hole. The high incidence of AGN radio cores in compact starbursts suggests little or no delay between the starburst phase and the onset of AGN activity.