Jet-accretion system in the nearby mJy Radio Galaxies

Abstract

It is generally thought that FRII Radio Galaxies host thin optically thick disks, while FRIs are powered by Advected Dominated Accretion Flows. The sources with an efficient engine are optically classified as High Excitation Radio Galaxies (HERGs) and those with an inefficient motor as Low Excitation Radio Galaxies (LERGs). Recently, the study of Radio Galaxies down to mJy fluxes has cast serious doubts on the LERG-FRI and HERG-FRII correspondence, revealing that many LERGs show FRII radio morphologies. The FR catalogs recently compiled by Capetti et al. (2017a,b) and Baldi et al. (2018) have allowed us to explore this issue in the local ($z\le 0.15$) mJy Universe. Our statistical study shows that the majority of nearby mJy objects are in a late stage of their life. FRII-LERGs appear more similar to the old FRI-LERGs than to the young FRII-HERGs. FRII-LERGs may be aged HERGs that, exhausted the cold fuel, have changed their accretion regime or a separate LERG class particularly efficient in launching jets. Exploiting the empirical relations which convert L$_{\rm [OIII]}$ and L$_{\rm 1.4~GHz}$ into accretion power and jet kinetic power, respectively, we observed that LERGs with similar masses and accretion rates seem to expel jets of different power. We speculate that intrinsic differences related to the black hole properties (spin and magnetic field at its horizon) can determine the observed spread in jet luminosity. In this view, FRII-LERGs should have the fastest spinning black holes and/or the most intense magnetic fluxes. On the contrary, compact LERGs (i.e. FR0s) should host extremely slow black holes and/or weak magnetic fields.