Investigating the Star-Formation Characteristics of Radio Active Galactic Nuclei

TL;DR

This study examines the star-formation rates of radio active galactic nuclei across different redshifts, revealing their dependence on galaxy mass and cosmic time, and comparing them to typical star-forming galaxies.

Contribution

It provides a comprehensive analysis of radio AGN star-formation characteristics up to redshift 3 using large, mass-complete samples, highlighting their evolving relation to the star-forming main sequence.

Findings

Radio AGNs are more likely in massive galaxies with low star-forming fractions.

The star-forming fraction of radio AGNs increases with redshift.

Radio AGNs have lower SFRs than star-forming galaxies at low redshift and high mass, but similar or higher SFRs at high redshift or low mass.

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 $M_{\star}$.