2023A&A...671A..32C

This paper is the seventh work in the MUse RAdio Loud Emission lines Snapshot (MURALES) project series, presenting the results of observations obtained with the VLT/MUSE integral field spectrograph of 3C radio sources. Here, we discuss the optical spectral properties of the nuclei of 26 objects with 0.3 < z < 0.82 (median redshift 0.51). At these redshifts, the Hα and [N II] emission lines are not covered by optical spectra and alternative diagnostic diagrams are needed to separate the different spectroscopic sub-classes. We derived a robust spectroscopic classification into high and low-excitation galaxies (HEGs and LEGs) by only using the ratios of emission lines in the rest-frame UV and the blue portion of the spectra. A key result of this study is that FR II LEGs are also found at the highest level of radio power (up to L_178 MHz_ ∼ 2 × 10³⁵ erg s^–1 Hz^–1), placing them among the most luminous radio sources in the Universe. Furthermore, their fraction within the FR II RG population does not strongly depend on radio luminosity. This suggests that the jet properties in powerful FR II radio sources do not depend on the accretion mode or on the structure of the accretion disk - as would otherwise be expected if the jet launching process were 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 active galactic nucleus (AGN) components.