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

TL;DR

This paper investigates the low-power end of the AGN radio luminosity function, establishing constraints on the break points to reconcile observed densities with theoretical limits, and linking these to the unified AGN model.

Contribution

It provides new constraints on the low-power break points of the AGN radio luminosity functions for LRP and BL Lac objects, connecting these to the unified model and jet physics.

Findings

The break in the RLF for LRP AGN is between 10^20.5 and 10^21.5 W/Hz.

The break in the RLF for BL Lacs is between 10^23.3 and 10^24.5 W/Hz.

The ratio of break luminosities supports a jet Doppler factor of about 10.

Abstract

We constrain the behavior 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 harbor them and this requires a break to a shallower slope. For LRP AGN we obtain P_br,LRP > 10^20.5 W/Hz at 1.4 GHz to limit their density to be smaller than that of elliptical galaxies with black hole masses M_BH > 10^7.5 solar masses. By combining this value with the limit derived by the observations the break must occur at P_br,LRP~10^20.5-10^21.5 W/Hz. For BL Lacs we find P_br,BLLAC > 10^23.3 W/Hz otherwise they would outnumber the density of weak-lined and compact radio sources, while the observations indicate P_br,BLLAC < 10^24.5 W/Hz. 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 P_br,LRP and P_br,BLLAC is ~10^3, as expected for a jet Doppler factor of ~10.