Magnetically arrested discs in FR I radio galaxies

A sample of 17 Fanaroff-Riley type I (FR I) radio galaxies constructed from the 3CR catalogue, which is characterized by edge-darkened radio structures, is studied. The optical core luminosities derived from Hubble Space Telescope observation are used to estimate the Eddington ratios which are found to be below 10(-3.4) for this sample. This is supported by the Baldwin-Phillips-Terlevich optical diagnostic diagrams derived with the spectroscopic observation of Telescopio Nazionale Galileo, suggesting that these sources are of low-ionization nuclear emission-line regions. It implies that the accretion in these FR I sources can be modelled as advection-dominated accretion flows (ADAFs). Given the low accretion rate, the predicted jet power with a fast-spinning black hole (BH) a = 0.95 in the Blandford-Znajek mechanics is lower than the estimated one for almost all the sources in our sample. Such powerful jets indicate the presence of magnetically arrested discs (MAD) in the inner region of the ADAF, in the sense that the magnetic fields in the inner accretion zone are strong. Moreover, we show that, even in the MAD scenario, the BH spins in the sample are most likely moderate and/or fast with a greater than or similar to 0.5.