An empirical model of the extragalactic radio background

Abstract

Aims. Radio observations provide a powerful tool for constraining the assembly of galaxies over cosmic time. Recent deep and wide radio continuum surveys have significantly improved our understanding of the radio emission properties of active galactic nuclei (AGNs) and star-forming galaxies (SFGs) across 0 < z < 4. These findings have allowed us to derive an empirical model of the radio continuum emission of galaxies, based on their star formation rates and the probability of their hosting radio AGNs. In this work, we verify how well this empirical model can reproduce the extragalactic radio background (ERB), which can provide new insights into the contribution to the ERB from galaxies of different masses and redshfits.Methods. We made use of the Empirical Galaxy Generator (EGG) code to generate a near-infrared (NIR) selected, flux-limited, multiwavelength catalog to mimic real observations. Then we assigned radio continuum flux densities to galaxies based on their star formation rates and the probability that they would host a radio-AGN of a specific 1.4 GHz luminosity. We also applied special treatments to reproduce the clustering signal of radio AGNs.Results. Our empirical model successfully recovers the observed 1.4 GHz radio luminosity functions (RLFs) of both AGN and SFG populations, as well as the differential number counts at various radio bands. The uniqueness of this approach also allows us to directly link the radio flux densities of galaxies to other properties, including redshifts, stellar masses, and magnitudes at various photometric bands. We find that roughly half of the radio continuum sources to be detected by the Square Kilometer Array (SKA) at z ∼ 4 − 6 will be too faint to be detected in the optical survey (r ∼ 27.5) carried out by Rubin Observatory.Conclusions. Unlike previous studies, which utilized (extrapolations of) RLFs to reproduce the ERB, our work starts from a simulated galaxy catalog with realistic physical properties. It has the potential to simultaneously and self-consistently reproduce physical properties of galaxies across a wide range of wavelengths, from the optical, NIR, and far-infrared (FIR) to radio wavelengths. Our empirical model can shed light on the contribution of different galaxies to the extragalactic background light and would greatly facilitate the design of future multiwavelength galaxy surveys.