A hidden Active Galactic Nuclei population: the first radio luminosity functions constructed by physical process

TL;DR

This study constructs radio luminosity functions based on physical processes, revealing a previously hidden population of low-luminosity AGN that impacts our understanding of galaxy evolution.

Contribution

It introduces a novel method to distinguish AGN from star formation in radio data, enabling the first physical-process-based radio luminosity functions.

Findings

Revealed a hidden AGN population at low radio luminosities.

Found the AGN population is 1.56 times larger than expected from galaxy classification.

Star formation estimates are reduced to 90% of previous expectations.

Abstract

Both star formation (SF) and Active Galactic Nuclei (AGN) play an important role in galaxy evolution. Statistically quantifying their relative importance can be done using radio luminosity functions. Until now these relied on galaxy classifications, where sources with a mixture of radio emission from SF and AGN are labelled as either a star-forming galaxy or an AGN. This can cause the misestimation of the relevance of AGN. Brightness temperature measurements at 144 MHz with the International LOFAR telescope can separate radio emission from AGN and SF. We use the combination of sub-arcsec and arcsec resolution imaging of 7,497 sources in the Lockman Hole and ELAIS-N1 fields to identify AGN components in the sub-arcsec resolution images and subtract them from the total flux density, leaving flux density from SF only. We construct, for the first time, radio luminosity functions by physical process, either SF or AGN activity, revealing a hidden AGN population at $L_{\textrm{144MHz}}$$<10^{24}$ W$\,$Hz$^{-1}$ . This population is 1.56$\pm$0.06 more than expected for $0.5<z<2.0$ when comparing to RLFs by galaxy classification. The star forming population has only 0.90$\pm$0.02 of the expected SF. These 'hidden' AGN can have significant implications for the cosmic star formation rate and kinetic luminosity densities.