Effects of interactions on spiral galaxies. I. Nuclear activity and star formation

We present results of spectrophotometry of the nuclei of 161 (mostly spiral) galaxies with bright companions, and emission-line imaging of 63 galaxies. The program includes a complete sample, unbiased as to morphological disturbance, and an Arp-atlas sample selected for visual evidence of tidal distortion. When compared to a similarly observed sample of isolated-spiral nuclei, both samples of interacting galaxies show significant excesses of nuclear emission. Both nonthermal (Seyfert) activity and nuclear star formation are enhanced, while low-ionization (LINER) nuclei are uncommon. The rate of nuclear star formation is significantly above average even in systems without noticeable tidal distortion in the outer disks, suggesting that the near-nuclear gas is only marginally stable in isolated galaxies. There is evidence that Seyfert nuclei are more frequent in very close pairs. Very disrupted systems possess significantly fewer Seyfert nuclei. These effects become more pronounced with increasing galaxy luminosity. Sizes of nuclear emission regions appear to be unchanged by interactions. The enhancement in nuclear star formation depends only weakly on pair separation over the range we have sampled. Our results suggest that nuclear phenomena are triggered by tidally induced influx of gas from the disk into the nuclear regions, rather than gas transfer between the galaxies