Galactic Smashups GALACTIC CRASHES Major and minor mergers have different effects on the galaxies involved. NGC 6621 and 6622, also known collectively as Arp 81 (right), exemplify the interaction of two evenly matched systems: the encounter has torn both galaxies apart. On the other hand, the large spiral NGC 5754 (below) is five times more massive than NGC 5752 (fuzz to its right) and survived their recent encounter relatively unscathed. Facing page: William Keel (University of Alabama) and Kirk Borne (now at George Mason University) endeavored to reconstruct these systems' major tidal structures (labeled images) in simulations of the interactions. In the simulation diagrams, the big dot marks the center of the contender that looped around, and orbits are orange where they passed behind the other galaxy from our perspective. NGC 5752/4 AND NGC 6621/2: NASA / ESA / HUBBLE HERITAGE (STSCI / AURA) & ESA / HUBBLE COLLABORATION / W. KEEL (UNIV. OF ALABAMA, TUSCALOOSA); SCHEMATICS: S&T: GREGG DINDERMAN, SOURCE: W. KEEL AND K. BORNE / ASTRONOMICAL JOURNAL 2003 18 September 2015 sky & telescope