Cosmic Evolution of Radio Selected Active Galactic Nuclei in the Cosmos Field

Abstract

We explore the cosmic evolution of radio luminous active galactic nuclei (AGNs) with low radio powers (L_(1.4 GHz) ≾ 5 × 10^(25) W Hz^(–1)) 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 AGNs, and its evolution with cosmic time assuming two extreme cases: (1) pure luminosity and (2) pure density evolution. The former and latter yield L_* α (1 + z)^(0.8 ± 0.1), and Φ_* α (1 + z)^(1.1 ± 0.1), respectively, both implying a fairly modest change in properties of low-radio-power AGNs since z = 1.3. We show that this is in stark contrast with the evolution of powerful (L_(1.4 GHz) > 5 × 10^25 W Hz^(–1)) radio AGN over the same cosmic time interval, constrained using the 3CRR, 6CE, and 7CRS radio surveys by Willot et al. 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 AGNs. Our findings suggest that high- and low-radio-power AGN activities are 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.

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