Radio-ExcessIRASGalaxies. IV. Optical Spectroscopy

This is the fourth in our series of papers investigating radio-excessgalaxies, which have radio emission associated with an active nucleus but whichdo not fit into the traditional categories of either radio-loud or radio-quietactive galaxies. In this paper, we present optical spectra of our sample ofFIR-luminous radio-excess galaxies. Optical emission line diagnostics are usedto determine the dominant source of the ionizing radiation. We find that radioexcess is an excellent indicator of the presence of an active nucleus: theradio-excess sample contains a much higher fraction of AGN than samplesselected on FIR luminosity alone, or using other criteria such as warm FIRcolors. Several objects have ambiguous classifications and are likely to becomposite objects with mixed excitation. The type of optical spectrum appearsto be associated with the radio-loudness: radio-loud objects may be more `pure'AGN than radio-intermediate objects. We find strong evidence for interactionbetween the radio plasma and the surrounding gas. The jet energy fluxes of theradio-excess objects, inferred from the [O III] luminosities, are lower than inpowerful radio sources, consistent with our previous results. We conclude thatthe jets of radio-intermediate sources are intrinsically weaker than those insources with more powerful radio emission. A significant fraction of the samplespectra show post-starburst stellar continuum, with A-star absorption lines,consistent with the large fraction of merging or disturbed host galaxies in thesample. The ages of the radio sources are significantly less than those of Astars indicating that, if the radio sources are associated with mergingactivity, there is a delay between the interaction and the initiation of theradio activity. (Abridged.)