Cosmic evolution of radio selected active galactic nuclei in the COSMOS field

TL;DR

This study investigates the evolution of low-power radio AGN up to redshift 1.3, revealing modest changes over cosmic time and contrasting with the evolution of powerful AGN, highlighting different triggering mechanisms and host galaxy properties.

Contribution

It provides the first detailed measurement of the cosmic evolution of low-power radio AGN luminosity function up to z=1.3, contrasting it with powerful AGN evolution and discussing implications for galaxy formation.

Findings

Low-power radio AGN show modest evolution since z=1.3.

High-power and low-power AGN evolve differently, indicating different fueling mechanisms.

Weak radio AGN are present in massive galaxies at z~1, influencing galaxy evolution.

Abstract

We explore the cosmic evolution of radio AGN with low radio powers (L_1.4GHz < 5\times10^25 W/Hz) out to z=1.3 using to-date the largest sample of ~600 low luminosity radio AGN at intermediate redshift drawn from the VLA-COSMOS survey. We derive the radio luminosity function for these AGN, and its evolution with cosmic time assuming two extreme cases: i) pure luminosity and ii) pure density evolution. The former and latter yield L_*\propto(1+z)^(0.8+/-0.1), and Phi*\propto (1+z)^(1.1+/-0.1), respectively, both implying a fairly modest change in properties of low radio-power AGN since z=1.3. We show that this is in stark contrast with the evolution of powerful (L_1.4GHz > 5\times10^25 W/Hz) radio AGN over the same cosmic time interval, constrained using the 3CRR, 6CE, and 7CRS radio surveys by Willott et al. (2001). We demonstrate that this can be explained through differences in black hole fueling and triggering mechanisms, and a dichotomy in host galaxy properties of weak and powerful AGN. Our findings suggest that high and low radio-power AGN activity is triggered in different stages during the formation of massive red galaxies. We show that weak radio AGN occur in the most massive galaxies already at z~1, and they may significantly contribute to the heating of their surrounding medium and thus inhibit gas accretion onto their host galaxies, as recently suggested for the `radio mode' in cosmological models.