Abstract
Abstract It is generally thought that FRII radio galaxies host thin optically thick disks, while FRIs are powered by advection-dominated accretion flows. 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. and Baldi et al. have allowed us to explore this issue in the local (z ≤ 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, having exhausted their cold fuel, have changed their accretion regime or are a separate LERG class particularly efficient in launching jets. Exploiting the empirical relations that convert L [O III] and L 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 powers. 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.
Highlights
Radio Galaxies (RGs) are historically divided in core-brightened FR I and bright edge-brightened FR II (Fanaroff & Riley 1974), on the basis of their extended radio morphology that approximately changes at a critical power P1.4 GHz ∼ 3 × 1025 W Hz−1. Ledlow & Owen (1996) refined the classification showing that GRANDI ET AL.the break between FRIs and FRIIs is a strong function of the host galaxy absolute magnitude (MR)
As FRIIs are generally associated to High Excitation Radio Galaxies (HERGs) and FRIs to Low Excitation Radio Galaxies (LERGs), it is almost natural to consider the nuclear engine as the main driver of the FRI-FRII dichotomy
Several studies show that radio galaxies with FRII morphologies preferentially host low efficient accretion flows at low flux densities (Capetti et al 2017a,b; Miraghaei & Best 2017)
Summary
Radio Galaxies (RGs) are historically divided in core-brightened FR I and bright edge-brightened FR II (Fanaroff & Riley 1974), on the basis of their extended radio morphology that approximately changes at a critical power P1.4 GHz ∼ 3 × 1025 W Hz−1. Ledlow & Owen (1996) refined the classification showing that. The less powerful radio galaxies (i.e. FRIs) host an inefficient hot thick flow, while the more powerful sources (i.e. FRIIs) have an efficiently accreting cold disk. Several studies show that radio galaxies with FRII morphologies preferentially host low efficient accretion flows (i.e. they are classified as LERGs) at low flux densities (Capetti et al 2017a,b; Miraghaei & Best 2017). A recent X-ray analysis of 3C radio galaxies (Macconi et al 2020) has shown that FRII-LERG nuclei have less cold gas, i.e. smaller column densities (NH) than FRII-HERGs. A possible suggestion is that a transition occurs from a thin disk to a thick flow in FRIIs when the cold fuel has been depleted.
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