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We have used the grism mode of the Near-Infrared Camera (NIRC) mounted at the Cassegrain focus of the W.M. Keck telescope to obtain spectra of the merging double nucleus galaxy Arp 220 in the 2.0-2.5 micron wavelength regime. These R$=80$ spectra have allowed us to measure the CO absorption band, as well as the atomic and molecular hydrogen emission lines over the central 6-7" at better than 1" resolution. The strong CO absorption yet weak Br $\gamma$ emission centered on the nuclei imply a very strong, young starburst is present within the central 2-3". The young stars involved in the burst, together with strong emission from warm dust heated by the burst, dominate the nuclear K-band spectra. We will discuss the importance of this burst for the overall energetics of Arp 220 by comparing the measured CO and Br $\gamma$ line strengths to various simple starburst models.
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