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

Abstract

Near-infrared spectra covering λ~2.0-2.5 µm 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°, 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 B γ 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 B γ 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 µm.

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