The frequency of active and quiescent galaxies with companions: Implications for the feeding of the nucleus

We analyze the idea that nuclear activity, either active galactic nuclei (AGNs) or star formation, can be triggered by interactions by studying the percentage of active, H II, and quiescent galaxies with companions. Our sample was selected from the Palomar survey and avoids selection biases faced by previous studies. This sample was split into five different groups, Seyfert galaxies, LINERs, transition galaxies, H II galaxies, and absorption-line galaxies. The comparison between the local galaxy density distributions of the different groups showed that in most cases there is no statistically significant difference among galaxies of different activity types, with the exception that absorption-line galaxies are seen in higher density environments, since most of them are in the Virgo Cluster. The comparison of the percentage of galaxies with nearby companions showed that there is a higher percentage of LINERs, transition galaxies, and absorption-line galaxies with companions than Seyfert and H II galaxies. However, we find that when we consider only galaxies of similar morphological types (elliptical or spiral), there is no difference in the percentage of galaxies with companions among different activity types, indicating that the former result was due to the morphology-density effect. In addition, only small differences are found when we consider galaxies with similar Hα luminosities. The comparison between H II galaxies of different Hα luminosities shows that there is a significantly higher percentage of galaxies with companions among H II galaxies with L(Hα) > 1039 ergs s-1 than among those with L(Hα) ≤ 1039 ergs s-1, indicating that interactions increase the amount of circumnuclear star formation, in agreement with previous results. The fact that we find that galaxies of different activity types have the same percentage of companions suggests that interactions between galaxies is not a necessary condition to trigger the nuclear activity in AGNs. We compare our results with previous ones and discuss their implications.