The contribution of active galactic nuclei to the microjansky radio population

Abstract

An 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, LX . 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 μJy AGN population will be dominated by obscured, radio-quiet Seyfert galaxies with log LX < 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 μJy. The star formation rates (SFRs) required are modest, only 3 M yr -1, assuming a constant rate with z and LX . A more observationally and theoretically motivated relationship, where the SFR (1 + z)1.76(log LX - 40)3.5, will also account for the observed counts. The μJy 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. © 2009. The American Astronomical Society.