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We constrain the behavior of the radio luminosity function (RLF) of twoclasses of active galactic nuclei (AGN) namely AGN of low radio power (LRP) andBL Lac objects. The extrapolation of the observed steep RLFs to low powerpredicts a space density of such objects that exceeds that of the sources thatcan harbor them and this requires a break to a shallower slope. For LRP AGN weobtain P_br,LRP > 10^20.5 W/Hz at 1.4 GHz to limit their density to be smallerthan that of elliptical galaxies with black hole masses M_BH > 10^7.5 solarmasses. By combining this value with the limit derived by the observations thebreak must occur at P_br,LRP~10^20.5-10^21.5 W/Hz. For BL Lacs we findP_br,BLLAC > 10^23.3 W/Hz otherwise they would outnumber the density ofweak-lined and compact radio sources, while the observations indicateP_br,BLLAC < 10^24.5 W/Hz. In the framework of the AGN unified model a lowluminosity break in the RLF of LRP AGN must correspond to a break in the RLF ofBL Lacs. The ratio between P_br,LRP and P_br,BLLAC is ~10^3, as expected for ajet Doppler factor of ~10.

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