The Contribution of Active Galactic Nuclei to the Microjansky Radio Population

TL;DR

This paper models the contribution of different types of Active Galactic Nuclei to the radio source counts at microjansky levels, highlighting the dominance of obscured, radio-quiet Seyfert galaxies and the role of star formation in host galaxies.

Contribution

It presents a comprehensive X-ray background synthesis model to predict AGN radio counts, including obscured and Compton-thick AGNs, and explores the impact of host galaxy star formation on observed fluxes.

Findings

Obscured, radio-quiet Seyfert galaxies dominate the microJy AGN population.

Star formation rates of about 3 solar masses per year can explain observed counts.

Different SFR models can account for the observed radio source counts.

Abstract

A X-ray background synthesis model is used to calculate the contribution of Active Galactic Nuclei (AGNs) to the 1.4 GHz number counts between 100 nJy and 10 mJy. The number counts are broken down into contributions from radio-quiet and radio-loud AGNs, obscured and unobscured AGNs, and for different ranges in redshift and 2-10 keV X-ray luminosity, L_X. Compton-thick AGNs are included, but only to the level required to fit the peak of the X-ray background. The predicted radio counts show that the microJy AGN population will be dominated by obscured, radio-quiet Seyfert galaxies with log L_X < 43, and spanning 0 < z <~ 3. However, depending on the exact relationship between the radio and X-ray luminosities in radio-quiet AGNs, additional radio flux due to star-formation within AGN host galaxies may be necessary in order to match the observed AGN counts at a flux density of ~50 microJy. The star-formation rates (SFR) required are modest, only ~3 Msun per year, assuming a constant rate with z and L_X. A more observationally and theoretically motivated relationship, where the SFR \propto (1+z)^{1.76}(log L_X-40)^{3.5}, will also account for the observed counts. The microJy AGN population will provide a very clean sample to trace the accretion and galactic star-formation histories of Seyfert galaxies over a significant fraction of cosmic time.