| Abstract | Optical, near-IR, and H I observations are presented of the peculiar interacting galaxies Arp 143 = VV 117 = NGC 2445/4. The bright blue optical knots which form a crescent shape in the disk of NGC 2445 lie on the leading edge of an H I density enhancement similar to that expected from the slightly off-center collision between two galaxies. The BVrJHK photometry of the bright blue knots is compared with models of the evolution of a starburst population and is found to be consistent with an extremely young stellar population (<= 10^7^ yr old). This time scale is short compared with the propagation of a density wave, suggesting that the star formation has only recently been triggered coherently in the wave. In addition to the blue knots in the disk of NGC 2445, there are some regions which appear most strongly in the IR. These regions are either (1) part of a patchy underlying density wave in the old stellar population of the disk or (2) very young starforming regions which are dominated by a small number of extremely luminous supergiants which have only recently evolved from the main sequence. The results are compared with the predictions of cloud-fluid models for the formation of a ring galaxy. Based on the models, we speculate that the overdensity will continue to rise as the wave propagates outward, and eventually the entire leading edge of the wave will undergo a starburst giving NGC 2445 the appearance of a classical ring galaxy. |
