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We present a study of the nuclear activity in a well-defined sample of themost isolated galaxies in the local Universe traced by their far-infrared (FIR)and radio continuum emission. We use the well-known radio continuum-FIRcorrelation to select radio-excess galaxies that are candidates to host anactive galactic nucleus (AGN), as well as the FIR colours to find obscured AGNcandidates. The existing information on nuclear activity in the V\'eron-Cettycatalogue and in the NASA Extragalactic Database are also used. A finalcatalogue of AGN-candidate galaxies has been produced. It contains 89 AGNcandidates and is publicly available on the AMIGA web page(http://www.iaa.csic.es/AMIGA.html). At most ~ 1.5 % of the galaxies shows aradio-excess with respect to the radio-FIR correlation, and this fraction evengoes down to less than 0.8 % after rejection of back/foreground sources. Wefind that the fraction of FIR colour selected AGN-candidates is ~ 28 % with alower limit of ~ 7 %. A comparison with the results from the literature showsthat the AMIGA sample has the lowest ratio of AGN candidates, both globally andseparated into early and late types. Field galaxies as well as poor cluster andgroup environments show intermediate values, while the highest rates of AGNcandidates are found in the central parts of clusters and in pair/mergerdominated samples. We conclude that the environment plays a crucial and directrole in triggering radio nuclear activity and not only via thedensity-morphology relation. Isolated, early-type galaxies show a particularlylow level of activity at radio wavelengths hence constituting the mostnurture-free population of luminous early-type galaxies.

The AMIGA sample of isolated galaxies. VII. Far-infrared and radio continuum study of nuclear activity