Investigating the Star-Formation Characteristics of Radio Active Galactic Nuclei

Abstract: The coevolution of supermassive black holes and their host galaxies represents a fundamental question in astrophysics. One approach to investigating this question involves comparing the star-formation rates (SFRs) of active galactic nuclei (AGNs) with those of typical star-forming galaxies. At relatively low redshifts ($z\lesssim 1$), radio AGNs manifest diminished SFRs, indicating suppressed star formation, but their behavior at higher redshifts is unclear. To examine this, we leveraged galaxy and radio AGN data from the well-characterized W-CDF-S, ELAIS-S1, and XMM-LSS fields. We established two mass-complete reference star-forming galaxy samples and two radio AGN samples, consisting of 1,763 and 6,766 radio AGNs, the former being higher in purity and the latter more complete. We subsequently computed star-forming fractions ($f_{\text{SF}}$; the fraction of star-forming galaxies to all galaxies) for galaxies and radio-AGN-host galaxies and conducted a robust comparison between them up to $z\approx3$. We found that the tendency for radio AGNs to reside in massive galaxies primarily accounts for their low $f_{\text{SF}}$, which also shows a strong negative dependence upon $M_{\star}$ and a strong positive evolution with $z$. To investigate further the star-formation characteristics of those star-forming radio AGNs, we constructed the star-forming main sequence (MS) and investigated the behavior of the position of AGNs relative to the MS at $z\approx0-3$. Our results reveal that radio AGNs display lower SFRs than star-forming galaxies in the low-$z$ and high-$M_{\star}$ regime and, conversely, exhibit comparable or higher SFRs than MS star-forming galaxies at higher redshifts or lower $M_{\star}$.