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Near--infrared $J$, $H$, and $K$ band images, and $K$ band spectra have been obtained for twenty radio--luminous starburst galaxies in order to develop an understanding of the evolution of star formation in interacting systems. The $J-H$ and $H-K$ colors are used to assess the importance of dust emission within each galaxy. The extinction and number of ionizing photons are estimated from observations of the Br$\gamma$ recombination line and published radio fluxes. In conjunction with stellar population models, the strength of the CO absorption features located between 2.32 and 2.40 $\mu$m provides an estimate of the fraction of the K band light emitted by young supergiants and the age of the burst. Optical rotation curves have been acquired to constrain the dynamical mass contained within the starburst region.
The star formation in these systems is extreme in comparison to that of galactic HII regions and M82. The massive stars formed in the burst produce 3 to 40 times as many ionizing photons as M82. Between 15 and 50\% of the observed K band emission is produced by hot dust. The amount of dust emission exceeds that expected from comparisons with galactic HII regions. Nearly all of the remaining light is produced by young supergiants. Evidence for an initial mass function (IMF) biased towards massive star formation is found in both the total stellar luminosity and infrared excess. Stellar population models show the slope of the IMF is shallower than that observed locally, and thus provide additional evidence for biased star formation.
These and other star formation properties have been determined as a function of the distance between the interacting nuclei. The IMF slopes are almost normal for widely separated systems, whereas merging galaxies appear to be more strongly biased towards massive star formation. However, the widely separated systems exhibit near--infrared colors and luminosities similar to the merging systems.
