NEAR-INFRARED SPECTROSCOPY OF THE ARP-220 NUCLEI - MEASURING THE NUCLEAR ROTATION

We report spatially resolved near-infrared spectroscopy at a resolutio n (lambda/Delta lambda) similar to 1000 of the double nucleus of Arp 2 20 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 r ecombination line is strongly peaked on the two nuclei, and the Pa bet a 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 redshifte d 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 rotatio n 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 t o be A(V) similar to 10 mag for the western nucleus and A(V) similar t o 13 mag for the eastern nucleus. The [Fe II] and H-2 lines show a dif ferent 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 t wo nuclei. The difference in spatial and velocity structure between th e [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 m ay have a significant contribution from extranuclear shocks related to either a starburst-driven wind or the ongoing merger process.