Accretion properties and jet mechanisms for the low-excitation radio galaxies

Context. Radio galaxies (RGs) are a subclass of active galactic nuclei, which are suggested to be the parent populations of blazars. Based on morphologies and radio powers, RGs can be classified as Fanaroff-Riley type 0 (FR 0s), I (FR Is), and II (FR IIs) RGs. According to the accretion-ejection paragram, RGs can be classified as low-excitation or high-excitation radio galaxies (LERGs or HERGs). Aims. We aim to compile a distance-limited (z < 0.15) sample of 431 LERGs (FR 0s, FR Is, and FR IIs) in order to discuss their jet-formation mechanism with the advection-dominated accretion-flow (ADAF) scenario, and compare their accretion properties with Fermi BL Lacertae objects (BL Lacs). Methods. We explored different jet mechanisms (Blandford-Znajek [BZ] model and a mixture of the BZ and Blandford-Payne hybrid model) within the framework of ADAF-type disc around a Kerr black hole for both LERGs and Fermi BL Lacs. Results. Based on standard assumptions on the accretion-ejection coupling in RGs, the maximum kinetic jet and accretion power for FR 0s, FR Is, and FR IIs can be explained by an ADAF with the pure BZ mechanism or hybrid jet mechanism. In addition, for one-third of the FR IIs, to account for their kinetic jet power that is higher than what is expected of the hybrid jet mechanism, the magnetic field could play an important role, for example as in the form of magnetisation-driven outflows or stronger magnetic structures observed in some BL Lacs with high jet powers. Conlcusions. Similarities between BL Lacs and LERGs (e.g. accretion-ejection and clustering properties) suggest that high-synchrotron-peaked BL Lacs could be the beamed counterparts of FR 0s, and a potential general unification between LERGs and BL Lacs populations is discussed. However, a complete sample of BL Lacs is needed to robustly compare the jet and accretion properties with those of LERGs in the future.