Mixed-morphology pairs as a breeding ground for active nuclei

Mixed-morphology pairs offer a simplification of the interaction equation that involves a gas-rich fast rotator paired with a gas-poor slow rotator. In past low-resolution IRAS studies it was assumed that the bulk of the far-infrared ( FIR) emission originated in the spiral component. However, our Infrared Space Observatory studies revealed a surprising number of early-type components with significant IR emission, some of which turned out to show active nuclei. This motivated us to look at the current statistics of active nuclei in mixed pairs using the radio-FIR continuum correlation as a diagnostic. We find a clear excess of early-type components with radio continuum emission and active nuclei. We suggest that they arise more often in mixed pairs via cross-fueling of gas from the spiral companion. This fuel is more efficiently channeled into the nucleus of the slow-rotating receptor. In a sample of 112 mixed-morphology pairs from the Karachentsev catalog, we find that about 25% - 30% of detected mixed pairs show a displacement from the radio-FIR relation defined by normal star-forming galaxies. The latter objects show excess radio continuum emission, while others extend the relation to unusually high radio and FIR flux levels. Many of the outliers or extreme emitters involve an early-type component with an active nucleus. The paired E/S0 galaxies in the sample exhibit a significant excess detection fraction and a marginal excess luminosity distribution compared to those of isolated unpaired E/S0 galaxies.