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

October 1995 • 1995ApJ...452..599L

Authors • Larkin, J. E. • Armus, L. • Knop, R. A. • Matthews, K. • Soifer, B. T.

Abstract • We report spatially resolved near-infrared spectroscopy at a resolution (λ/Δλ) ∼ 1000 of the double nucleus of Arp 220 in the Paβ, Brγ, [Fe II] (λ = 1.2567 μm), and H₂ (1-0 S1) (λ = 2.1218 μm) emission lines. The Brγ hydrogen recombination line is strongly peaked on the two nuclei, and the Paβ is centered on the western nucleus with emission across the eastern nucleus. Both lines show a velocity separation of ∼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 × 10⁹ M_sun. The extinction determined from the ratio of the Paβ and Brγ lines is found to be A_V ∼ 10 mag for the western nucleus and A_V ∼ 13 mag for the eastern nucleus. The [Fe II] and H₂ lines show a different morphology than the hydrogen recombination lines, particularly the H₂ line which shows significant flux between the two nuclei. The [Fe II] and H₂ lines also do not show velocity splitting between the two nuclei. The difference in spatial and velocity structure between the [Fe II], H₂, and ionized hydrogen lines implies multiple excitation processes are at work within the nucleus. The [Fe II] and H₂ 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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