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

Context. This paper is part of a series involving the AMIGA project (Analysis of the Interstellar Medium of Isolated GAlaxies). This project provides a statistically-significant sample of the most isolated galaxies in the northern sky.Aims. We present a study of the nuclear activity in a well-defined sample of the most isolated galaxies (total sample: n = 1050, complete subsample: n = 719) in the local Universe traced by their far-infrared (FIR) and radio continuum emission.Methods. We use the well-known radio continuum-FIR correlation to select radio-excess galaxies that are candidates to host an active galactic nucleus (AGN), as well as the FIR colours to find obscured AGN-candidates. We also used the existing information on nuclear activity in the Véron-Cetty catalogue and in the NASA Extragalactic Database.Results. A final catalogue of AGN-candidate galaxies has been produced that will provide a baseline for studies on the dependence of activity on the environment. Our sample is mostly radio quiet, consistent with its high content of late-type galaxies. At most ~1.5% of the galaxies show a radio excess with respect to the radio-FIR correlation, and this fraction even goes down to less than 0.8% after rejection of back/foreground sources using FIRST. We find that the fraction of FIR colour selected AGN-candidates is ~28% with a lower limit of ~7% Our final catalogue contains 89 AGN candidates and is publicly available on the AMIGA web page (http://www.iaa.csic.es/AMIGA.html). A comparison with the results from the literature shows that the AMIGA sample has the lowest ratio of AGN candidates, both globally and separated into early and late types. Field galaxies as well as poor cluster and group environments show intermediate values, while the highest rates of AGN candidates are found in the central parts of clusters and in pair/merger dominated samples. For all environments, early-type galaxies show a higher ratio of radio-excess galaxies than late types, as can be expected, since massive elliptical galaxies are the usual hosts of powerful radio continuum emission. Conclusions. We conclude that the environment plays a crucial and direct role in triggering radio nuclear activity and not only via the density-morphology relation. Isolated, early-type galaxies show a particularly low level of activity at radio wavelengths hence constituting the most nurture-free population of luminous early-type galaxies.L.V.M., U.L., S.L., J.S. and J.S.M. are partially supported by DGI Grant AYA 2005-07516-C02-01 and Junta de Andalucía (Spain). S.L. was partially supported by an Averroes fellowship contract from the Junta de Andalucía. UL is supported by a Ramon y Cajal fellowship contract and by the DGI Grant ESP2003-00915. J.S.M. is supported by a fellowship from the Secretaría de Estado de Educación y Universidades