Radio properties of FIR-megamaser nuclei

¹ ASTRON, Westerbork Observatory, PO Box 2, 7990 AA Dwingeloo, The Netherlands e-mail: baan@astron.nl
² University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK e-mail: hrk@astro.ox.ac.uk

Received: 8 November 2004
Accepted: 7 December 2005

Abstract

Aims.Radio data on the nuclear emissions have been used to characterize the dominant nuclear activity in a sample of FIR (ultra-) luminous galaxies and the subgroup of known OH Megamasers. This study complements an earlier study of the optical classification of these Megamaser nuclei.Methods.Classification of the radio activity in the nuclei is based on three critical parameters: the radio brightness temperature, the radio spectral index, and the ratio of FIR and radio fluxes. A first method gives equal weight to the three parameters and a second method uses a weighted function to classify the nuclei.Results.The present sample shows that only 43% of the sample shows some – weak or strong – AGN characteristics. About 66% of the OH-MM sample and 81% of the non-OH-MM sample can be actually classified as Starburst-dominated sources. Radio diagnostic diagrams using these diagnostic parameters show a continuous distribution ranging between AGN-dominated and SBN-dominated sources. The diagnostic diagrams also support the notion that AGNs and starbursts coexist in the nuclei.Conclusions.A comparison of the radio and optical classifications shows a consistency in the extreme cases of clear SBN and AGNs. A significant part of the sources with optical AGN-like activity have an SBN classification in the radio. The discrepant classifications are discussed in order to arrive at a final classification of the dominant power source in the nucleus.