2010ApJ...718..368L

We studied the active galactic nucleus (AGN) radio emission from a compilation of hard X-ray-selected samples, all observed in the 1.4 GHz band. A total of more than 1600 AGNs with 2-10 keV de-absorbed luminosities higher than 10⁴² erg/s/cm2 were used. For a sub-sample of about fifty z ≲ 0.1 AGNs, it was possible to reach ∼80% of radio detections and therefore, for the first time, it was possible to almost completely measure the probability distribution function of the ratio between the radio and the X-ray luminosity R_X= log(L_1.4/L_X), where L_1.4/L_X= νL_ν(1.4 GHz)/L_X (2-10 keV). The probability distribution function of R_X was functionally fitted as dependent on the X-ray luminosity and redshift, P(R_X|L_X, z). It roughly spans over six decades (-7< R_X←1) and does not show any sign of bi-modality. The result is that the probability of finding large values of the R_X ratio increases with decreasing X-ray luminosities and (possibly) with increasing redshift. No statistically significant difference was found between the radio properties of the X-ray absorbed (N_H>10²²/cm2) and un-absorbed AGNs. Measurement of the probability distribution function of R_X allowed us to compute the kinetic luminosity function and the kinetic energy density which, at variance with that assumed in many galaxy evolution models, is observed to decrease by about a factor of 5 at redshift below 0.5. About half of the kinetic energy density results in being produced by the more radio quiet (R_X←4) AGNs. In agreement with previous estimates, the AGN efficiency ε_kin in converting the accreted mass energy into kinetic power ({L_K=εkin{dot}m c^2}) is, on average, ε_kin=~ 5x10^–3. The data suggest a possible increase of ε_kin at low redshifts.