Radio-excess IRAS galaxies. III. Radio properties

This is the third in a series of papers discussing the nature of radio-excess IRAS galaxies. This paper investigates in more detail the radio source properties of the radio-excess galaxies that are luminous in the far infrared (FIR). Spectral energy distributions derived from data from the Australia Telescope Compact Array confirm that the radio spectral energy distributions of the FIR-luminous radio-excess galaxies are mostly steep or show a turnover at gigahertz frequencies. VLBI snapshot data indicate that most sources are compact and thus resemble compact steep-spectrum (CSS) or gigahertz-peaked spectrum (GPS) sources. Images of two representative FIR-luminous radio-excess galaxies are analyzed. Their morphologies appear disrupted and do not resemble typical CSS/GPS sources with symmetric double lobes. Synchrotron cooling timescales are derived for several sources and are found to be ≲10⁶ yr. The jet energy fluxes inferred are 10 ³⁶-10³⁷ W, an order of magnitude lower than typical radio-loud CSS/GPS sources. A dynamical model of a jet-red lobe is used to derive ages ∼10⁴ yr and jet energy fluxes of ∼10³⁷ W for the sources observed with VLBI. Expansion velocities less than 0.01c are derived for the lobes using this model. Thus, the ages of the FIR-luminous radio-excess galaxies are similar to young radio-loud sources, but the jet energy fluxes and expansion velocities are significantly lower than CSS/GPS sources. We conclude that the lower jet energy fluxes are responsible for their observed weaker radio powers when compared with CSS/GPS sources. We do not expect that the radio-excess sources will evolve into classical radio galaxies in the manner envisaged for CSS/GPS sources.