NEAR-INFRARED SPECTRA OF ARP-220 - SPATIALLY-RESOLVED CO ABSORPTION IN THE INNER KILOPARSEC

Near-infrared spectra covering lambda similar to 2.0 - 2.5 mu m with a spectral resolution of R similar to 80 and a spatial resolution of 0. 7 '' have been obtained from two long slits oriented at a position ang le of 96 degrees, centered on and 2.0 '' south of the two near-infrare d nuclei in the ultraluminous infrared galaxy Arp 220. These data have allowed us to determine the strengths of the stellar CO absorption fe ature and the B gamma emission line over the central 7 arcsec (2.5 kpc ) on scales of similar to 250 pc. The two near-infrared nuclei have ve ry strong stellar atmospheric CO absorption coupled with very red H - K colors, implying significant contributions (40%-50%) to the K-band c ontinua from young, red supergiant stars formed in a recent burst of s tar formation. In contrast to the strong nuclear CO absorption, the eq uivalent width of the By emission line is weak, and peaked at the posi tion of the eastern infrared nucleus where the equivalent width is sim ilar to 1.0 nm. There is no detectable By emission in the spectra extr acted from the southern slit. We suggest that the youngest part of the starburst (t similar to 7 - 8 Myr old) is confined to the Arp 220 nuc lei, while the surrounding region (out to at least 2.0 '' - 2.5 '' rad ius) is dominated by older stars (similar to 10(8) yr) associated with a previous burst. Together these components are able to account for l ess than 10% of the bolometric luminosity of Arp 220. The dominant ene rgy source in Arp 220 apparently remains hidden even at 2.2 mu m. (C) 1995 American Astronomical Society.