Looking below the floor: Constraints on the AGN radio luminosity functions at low power

Abstract

We constrain the behaviour of the radio luminosity function (RLF) of two classes of active galactic nuclei (AGN) namely AGN of low radio power (LRP) and BL Lac objects. The extrapolation of the observed steep RLFs to low power predicts a space density of such objects that exceeds that of the sources that can harbour them and this requires a break to a shallower slope. For LRP AGN, we obtain Pbr,LRP ≳ 1020.5WHz-1 at 1.4 GHz to limit their density to be smaller than that of elliptical galaxies with black hole masses MBH > 107.5M⊙. By combining this value with the limit derived by the observations the break must occur at Pbr,LRP ~ 1020.5-1021.5WHz-1. For BL Lacs, we find Pbr,BLLAC ≳ 1023.3WHz-1 otherwise they would outnumber the density of weak-lined and compact radio sources, while the observations indicate Pbr,BLLAC ≲ 1024.5WHz-1. In the framework of the AGN unified model, a low luminosity break in the RLF of LRP AGN must correspond to a break in the RLF of BL Lacs. The ratio between Pbr, LRP and Pbr, BLLAC is ~103, as expected for a jet Doppler factor of ~10.