SESSIONS I and II: OVERVIEW

William Keel

University of Alabama

I set the stage for discussion of the stellar populations in interacting galaxies by looking back over the slow development of our understanding of these systems. From early anecdotal collections, to systematic cataloging, and finally to increasingly sophisticated n- body calculations, we have seen how gravity in distributed systems can produce the stunning variety of structures we see. At the same time, measures across the spectrum have made it clear that galaxy interactions are linked to star formation, albeit with the physical mechanisms much less clear. Improved data sets, including HST imaging, deep IR data, and large samples with well-defined selection criteria, have started to reveal correlations with dynamical parameters pointing to detailed histories of starbirth during collisions. Finally, the merger hypothesis for elliptical galaxies has broadened into seeing interactions and mergers as important parts of the overall evolution of galaxies. The connection becomes more important as we look to higher redshift, where more frequent interactions can drive the evolution of galaxies in multiple ways.

James L. Higdon

Georgia Southern University

Besides being photogenic, ring galaxies illustrate how ordinary spiral galaxies can be transformed through the action of a particularly simple collision. The ISM of ring galaxies can experience markedly different range of environments and evolution compared with a typical spiral's. They can as a result provide interesting perspectives into the triggering and regulation of star formation on large scales. We present new high angular resolution radio, millimeter, infrared, and optical observations of some classic ring galaxies, including AM0644-741, and use these to examine gravitational instabilities and star formation, the primary factors influencing the ring's molecular content, and evidence of departures from the standard "Schmidt" star formation law.

J. S. Gallagher

University Wisconsin-Madison

Present day galaxy interactions often involve pairs which are mismatched in mass--small systems perturbing or merging with giants. This talk builds from an overview of the interactions associated with M81, where ~250 million years ago NGC 3077 made a close passage by the M81 spiral, which was closely followed by M82's near miss. Studies of young stellar populations reveal the widespread impact of this "mild" interaction: the formation of a possible tidal dwarf system (Ho IX), significant star formation along the apparently unbound HI bridges, and the spectacular M82 and NGC3077 starbursts. M81 and other systems experiencing "minor interactions" demonstrate how even grazing collisions between galaxies have profound evolutionary implications, while also providing useful laboratories for studies of galaxy building processes. A further examination of low level galaxy conflicts ranging from M51 to various Arp systems reveals how these collisional processes can enhance spiral structure, potentially helping to extend outer galactic disks, while also profoundly changing the structures of the low mass perturbers, often towards earlier and more centrally concentrated morphologies.