Collisions between galaxies are thought to be common, with most galaxies probably involved in one or more encounters during their lifetimes. Our own Milky Way, for instance, is bound by gravity to our neighbor, the Andromeda galaxy, which we are approaching at a speed of about 50 kilometers per second, perhaps to meet in another few billion years. Astronomers believe that galaxy-galaxy interactions stimulate the vigorous formation of stars because gravitational effects during the encounters induce interstellar gas to condense into stars. These starbursts in turn light up the galaxies, especially at infrared wavelengths, making some systems hundreds or even thousands of times brighter than the Milky Way -- at least while the starbursts are underway. Many of the massive stars that are produced become supernovae whose explosive deaths enrich the environment with carbon, oxygen, and all the other elements that are essential for life but absent in primordial materials. Luminous galaxies are important not only in shedding light on how galaxies evolve, form stars, and seed the interstellar medium, but because they are bright and can be seen across cosmological distances where they act as lanterns for the relatively early universe.
The details of galaxy collisions are only vaguely understood. One reason is that most interactions involve galaxies of unequal size and brightness. Yet these are the pairs hardest to identify because there are so many faint galaxies in the sky and care is needed to identify ones physically associated with each other, rather than just coincidentally in the field of view. SAO astronomer Margaret Geller and a student, Deborah Woods, have just published the results of their study of 1204 nearby interacting galaxies in which one galaxy is comparatively faint; for comparison, they also study 2409 interacting galaxies in which the pairs are more nearly equal in brightness, and a field sample of over 65,000 other galaxies as a control. These large samples allow the scientists to draw statistically significant conclusions about star formation in interactions. The astronomers report that, in every brightness range they measured, interacting galaxies have higher levels of star formation than non-interacting ones, with the less luminous galaxies in the pair showing a relatively higher rate of star formation. The results are important because they demonstrate convincingly that gravitational forces trigger bursts of star formation, even in systems of unequal brightness.