THE INFLUENCE OF GALAXY INTERACTIONS ON NUCLEAR ACTIVITY (INFRARED, PHOTOMETRY).

Abstract

This dissertation examines empirically the effects of gravitational interactions between galaxies on their respective nuclei with the primary motivation of investigating the possible connection between such interactions and the presence of vigorous nuclear activity. To carry out this work, ground based near and mid-infrared observations, along with the mid- and far infrared data of IRAS of a statistically complete sample of interacting galaxies drawn from the Catalog of Isolated Pairs of Galaxies in the Northern Hemisphere (Karachentsev 1972) were utilized. Also performed were detailed probes of four known active, interacting and possible interacting systems, which made use of infrared multiaperture photometry and spectrophotometry. Comparison of the nuclear infrared properties of the interacting galaxies with samples of non-interacting galaxies shows that abnormal activity, characterized by excess 10 μm emission or extremely red or blue near infrared colors, is much more common in the interacting systems. In particular, a population of nuclei with extremely luminous 10 μm emission appears to be unique to the interacting sample. The in-depth studies of individual active systems have revealed extended 3.3 μm emission around the nucleus of NGC 7469, implying the presence of an extended heating source. The most plausible such source are the hot stars associated with star forming regions surrounding the Seyfert nucleus. The extraordinary Seyfert galaxy Mrk 231 may also be the site for powerful star formation, as evidenced by the extreme luminosity of the extended stellar system containing the Seyfert nucleus. This star formation may have resulted from the assimilation of a small satellite galaxy with a giant elliptical, or a collision between smaller systems. Tools similar to those used to probe NGC 7469 and Mrk 231 were used to investigate the interacting systems of NGC 6240 and Arp 220. These two objects are found to be the sites of star formation on an unprecedented scale, possibly involving up to 10¹⁰ Mₒ of material. The evidence suggesting the presence of these "super starbursts" includes an exceedingly luminous extended stellar component seen at 2 μm, a large population of red supergiants implied by deep stellar CO absorption, strong 3.3 μm emission and 9.7 μm silicate absorption, and an extended luminosity source indicated by the extent of the 10 μm emission. Both of these galaxies exhibit prominent lines of shocked molecular hydrogen which may arise in the collision of their interstellar clouds in an ongoing interaction.