2006AJ....132...27B

This is the fourth in our series of papers discussing the nature of radio-excess galaxies, which have radio emission associated with an active nucleus but do not fit into the traditional categories of either radio-loud or radio-quiet active galaxies. In this paper we present optical spectra of our sample of far-infrared (FIR)-luminous radio-excess galaxies. Optical emission-line ratio diagnostics are used to determine the dominant source of the ionizing radiation. We find that radio excess is an excellent indicator of the presence of an active nucleus. The radio-excess sample contains a much higher fraction of active galactic nuclei (AGNs) than samples selected on FIR luminosity alone or using other criteria such as warm FIR colors. Several objects have ambiguous classifications and are likely to be composite objects with mixed excitation. The type of optical spectrum appears to be associated with the radio loudness: our results suggest that radio-loud objects may be more ``pure'' AGNs than radio-intermediate objects. We find strong evidence for interaction between the radio plasma and the surrounding gas. Broad, structured optical emission lines are observed, and a relative blueshift is measured between the [O III] λ5007 and Hα lines in several sources. Jet energy fluxes are inferred from the [O III] λ5007 luminosities using a shock model for the interaction between the radio jet and the line-emitting gas. The jet energy fluxes of the radio-excess objects are lower than in powerful radio sources, consistent with our previous results. We conclude that the jets of radio-intermediate sources are intrinsically weaker than those in sources with more powerful radio emission. A significant fraction of the sample spectra show poststarburst stellar continua with A star absorption lines. Poststarburst stellar populations are consistent with the large fraction of merging or disturbed host galaxies in the sample. The ages of the radio sources are significantly less than those of A stars, indicating that, if the radio sources are associated with merging activity, there is a delay between the interaction and the initiation of the radio activity.