Near-Infrared Spectra of ARP 220: Spatially Resolved CO Absorption in the Inner Kiloparsec

Near-infrared spectra covering λ ~ 2.0-2.5 microns with a spectral resolution of R ~ 80 and a spatial resolution of 0.7" have been obtained from two long slits oriented at a position angle of 96^deg^, centered on and 2.0" south of the two near-infrared nuclei in the ultraluminous infrared galaxy Arp 220. These data have allowed us to determine the strengths of the stellar CO absorption feature and the Bγ emission line over the central 7 arcsec (2.5 kpc) on scales of ~250 pc. The two near-infrared nuclei have very strong stellar atmospheric CO absorption coupled with very red H-K colors, implying significant contributions (40%-50%) to the K-band continua from young, red supergiant stars formed in a recent burst of star formation. In contrast to the strong nuclear CO absorption, the equivalent width of the B γ emission line is weak, and peaked at the position of the eastern infrared nucleus where the equivalent width is ~1.0 nm. There is no detectable Bγ emission in the spectra extracted from the southern slit. We suggest that the youngest part of the starburst (t ~ 7-8 Myr old) is confined to the Arp 220 nuclei, while the surrounding region (out to at least 2.0"-2.5" radius) is dominated by older stars (~10^8^ yr) associated with a previous burst. Together these components are able to account for less than 10% of the bolometric luminosity of Arp 220. The dominant energy source in Arp 220 apparently remains hidden even at 2.2 microns.