The MURALES survey. VII. Optical spectral properties of the nuclei of 3C radio sources at 0.3<z<0.82

Abstract: This seventh paper of the MUse RAdio Loud Emission lines Snapshot (MURALES) project presents the results of the observations obtained with the VLT/MUSE integral field spectrograph of 3C radio sources and discusses the optical spectral properties of the nuclei of 26 objects with 0.3<z<0.82 (median redshift 0.51). At these redshifts the H$\alpha$ and [NII] emission lines are not covered by optical spectra and alternative diagnostic diagrams are needed to separate the different spectroscopic sub-classes. We derive a robust spectroscopic classification into high and low excitation galaxies (HEGs and LEGs) by only using ratios of emission lines in the rest frame UV and blue portion of the spectra. A key result is that FRII/LEGs are found also at the highest level of radio power (up to L$_{178} \sim 2\times 10^{35}$ erg/s/Hz), among the most luminous radio sources in the Universe. Furthermore, their fraction within the FRII RGs population does not strongly depend on radio luminosity. This suggests that the jet properties in powerful FRII radio sources do not depend on the accretion mode and on the structure of the accretion disk as expected if the jet launching process is due to the extraction of the rotational energy of the supermassive black hole. The alternative possibility of recurrent transitions between a LEG and a HEG phase is disfavored based on the variation timescales of the various AGN components.