Near-Infrared Spectroscopy of the ARP 220 Nuclei: Measuring the Nuclear Rotation

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Abstract
We report spatially resolved near-infrared spectroscopy at a resolution (lambda/Delta lambda) similar to 1000 of the double nucleus of Arp 220 in the Pa beta, Br gamma, [Fe II] (lambda = 1.2567 mu m), and H-2 (1-0 S1) (lambda = 2.1218 mu m) emission lines. The Br gamma hydrogen recombination line is strongly peaked on the two nuclei, and the Pa beta is centered on the western nucleus with emission across the eastern nucleus. Both lines show a velocity separation of similar to 200 km s(-1) between the eastern and western nuclei, the former being redshifted relative to the latter. Both the magnitude and the direction of this velocity shift are consistent with recent CO millimeter measurements of larger scale gas motions over the central 2 ''. The implied rotation requires a central mass greater than 1.5 x 10(9) M.. The extinction determined from the ratio of the Pa beta and Br gamma lines is found to be A(V) similar to 10 mag for the western nucleus and A(V) similar to 13 mag for the eastern nucleus. The [Fe II] and H-2 lines show a different morphology than the hydrogen recombination lines, particularly the H, line which shows significant flux between the two nuclei. The [Fe II] and H-2 lines also do not show velocity splitting between the two nuclei. The difference in spatial and velocity structure between the [Fe II], H-2, and ionized hydrogen lines implies multiple excitation processes are at work within the nucleus. The [Fe II] and H-2 lines may have a significant contribution from extranuclear shocks related to either a starburst-driven wind or the ongoing merger process.

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