Extreme Star Formation in the Interacting Galaxy Arp 299 (IC 694 + NGC 3690)^*

We present a comprehensive study of the star formation properties of the infrared luminous galaxy Arp 299 (IC 694 + NGC 3690). The observations include Hubble Space Telescope (HST) NICMOS imaging and Multiple Mirror Telescope optical and near-infrared spectroscopy together with HST archival data. We correct the galaxy parameters for extinction and use the results as boundary conditions for evolutionary starburst models. These models and other arguments show that Arp 299 has been going through a broad variety of interaction-induced star formation for the last ~15 Myr. In addition to nuclear starbursts in the two colliding galaxies, there are nearby luminous star-forming regions that may be the result of molecular clouds breaking up and starting to form massive stars as they approach the nuclear potential. Two regions near the interaction region have very recently formed massive stars (~4 Myr old). One of these regions in particular appears to have undergone a starburst of very short duration, in contrast with the 5-10 Myr durations that are typical of nuclear events. These regions will probably form gravitationally bound new dwarf galaxies, although they are close enough to the more massive original galaxies that they will probably eventually be subsumed into them. In addition, we find 19 H II regions at least as energetic as 30 Doradus, and 21 older star clusters that likely are the products of similar H II regions formed in the past. This population of supergiant H II regions is unprecedented in normal galaxies and emphasizes that the effects of the interaction propagate through the entire pair of galaxies.