The VLA-COSMOS 3 GHz Large Project: Star formation properties and radio luminosity functions of AGN with moderate-to-high radiative luminosities out to z∼ 6

¹ University of Zagreb, Physics Department, Bijenička cesta 32, 10002 Zagreb, Croatia
e-mail: lceraj@phy.hr
² Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio – Bologna, Via Piero Gobetti 93/3, 40129 Bologna, Italy
⁴ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁵ Astronomisches Institut, Ruhr-Universität Bochum, Universitätsstrasse 150, 44801 Bochum, Germany
⁶ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa

Received: 24 July 2018
Accepted: 30 October 2018

Abstract

We have studied a sample of 1604 moderate-to-high radiative luminosity active galactic nuclei (HLAGN) selected at 3 GHz within the VLA-COSMOS 3 GHz Large Project. These were classified by combining multiple AGN diagnostics: X-ray data, mid-infrared data and broadband spectral energy distribution fitting. We decomposed the total radio 1.4 GHz luminosity (L_{1.4 GHz, TOT}) into the emission originating from star formation and AGN activity by measuring the excess in L_{1.4 GHz, TOT} relative to the infrared-radio correlation of star-forming galaxies. To quantify the excess, for each source we calculated the AGN fraction (f_AGN) defined as the fractional contribution of AGN activity to L_{1.4 GHz, TOT}. The majority of the HLAGN, (68.0 ± 1.5)%, are dominated by star-forming processes (f_AGN ≤ 0.5), while (32.0 ± 1.5)% are dominated by AGN-related radio emission (0.5 <  f_AGN ≤ 1). We used the AGN-related 1.4 GHz emission to derive the 1.4 GHz AGN luminosity functions of HLAGN. By assuming pure density and pure luminosity evolution models we constrained their cosmic evolution out to z ∼ 6, finding Φ^*(z)∝(1 + z)^{(2.64 ± 0.10)+(−0.61 ± 0.04)z} and L^*(z)∝(1 + z)^{(3.97 ± 0.15)+(−0.92 ± 0.06)z}. These evolutionary laws show that the number and luminosity density of HLAGN increased from higher redshifts (z ∼ 6) up to a maximum in the redshift range 1 <  z <  2.5, followed by a decline toward local values. By scaling the 1.4 GHz AGN luminosity to kinetic luminosity using the standard conversion, we estimate the kinetic luminosity density as a function of redshift. We compare our result to the semi-analytic models of radio mode feedback, and find that this feedback could have played an important role in the context of AGN-host co-evolution in HLAGN which shows evidence of AGN-related radio emission (f_AGN >  0).