The Frequency of Active and Quiescent Galaxies with Companions: Implications for the Feeding of the Nucleus

We analyze the idea that nuclear activity, either AGN or star formation, canbe triggered by interactions, studying the percentage of active, HII andquiescent galaxies with companions. Our sample was selected from the Palomarsurvey, and avoids selection biases faced by previous studies. The comparisonbetween the local galaxy density distributions showed that in most cases thereis no statistically significant difference among galaxies of different activitytypes. The comparison of the percentage of galaxies with nearby companionsshowed that there is a higher percentage of LINERs, transition, and absorptionline galaxies with companions than Seyferts and HII galaxies. However, we findthat when we consider only galaxies of similar morphological types (ellipticalsor spirals), there is no difference in the percentage of galaxies withcompanions among different activity types, indicating that the former resultwas due to the morphology-density effect. Also, only small differences arefound when we consider galaxies with similar Halpha luminosities. Thecomparison between HII galaxies of different Halpha luminosities shows thatthere is a significantly higher percentage of galaxies with companions amongthe higher luminosity HII galaxies, indicating that interactions increase theamount of circumnuclear star formation, in agreement with previous results. Thefact that we find that galaxies of different activity types have the samepercentage of companions, suggests that interactions between galaxies is not anecessary condition to trigger the nuclear activity in AGNs. We compare ourresults with previous ones and discuss their implications. (abridged)Comment: 30 pages, including 6 figures and 3 tables. To appear in The Astronomical Journal, November issu