Arp 220: A Circumnuclear Polar Ring as an Alternative to a Double Nucleus?

Abstract

Recent millimeter interferometer observations of Arp 220 have yielded ~05 resolution images showing two strong concentrations of millimeter radio continuum and CO line emission embedded in a larger molecular gas disk (r ~ 1 kpc). The interferometer observations also revealed a complex velocity field with steep velocity gradients across each of the flux concentrations of Δv ~ 500 km s-1 within r = 03. The directions of these gradients are not aligned with each other or with that of the outer gas disk. This led to the conclusion that the two emission peaks represent either double nuclei with their own gas disks (r ~ 100 pc), which are counterrotating with respect to each other and rotate around the dynamical center of the system, or that they are two hot spots within a nuclear molecular gas disk. The overall structure of the molecular gas distribution and the corresponding complex velocity field, however, are highly symmetric, except for the unequal brightnesses of the two flux concentrations. This fact and similarities to the distribution and kinematics of the molecular gas in the central 2'' of NGC 1068 motivated us to try describing the Arp 220 nucleus as a warped molecular gas disk in which the peaks in the flux and line width distributions arise as a result of crowding of inclined circular orbits. Hubble Space Telescope (HST) images of the dust lanes flaring toward larger radii provide further support for a warped gas disk. The final model is surprisingly successful in explaining the CO line flux distribution, the velocity field, and position-velocity diagrams across the central flux concentrations. The model represents a strong 90° warp of the gas orbits out of the principal plane forming a circumnuclear polar ring and reproduces well the structures in HST near-IR color and continuum maps. The large number of young stars recently formed out of the warped, polar-ring gas implies that those stars will have kinematic properties similar to that of the molecular gas. We suggest that the red near-IR source at the center of the polar ring is the true nucleus of Arp 220. The success of the model implies that the two flux concentrations are not necessarily counterrotating nuclei, but rather the result of a combination of hot spots and orbit crowding due to the warp. The warp could be a direct consequence of the recent merger event indicated by tidal tails in optical images.