Morphology and Kinematics of Warm Molecular Gas in the Nuclear Region of
  Arp 220 as Revealed by ALMA
  We present Atacama Large Millimeter Array (ALMA) Cycle-0 observations of the
CO J = 6-5 line in the advanced galaxy merger Arp 220. This line traces warm
molecular gas, which dominates the total CO luminosity. The CO emission from
the two nuclei is well resolved by the 0.39" x 0.22" beam and the exceptional
sensitivity and spatial/spectral resolution reveal new complex features in the
morphology and kinematics of the warm gas. The line profiles are asymmetric
between the red and blue sides of the nuclear disks and the peak of the line
emission is offset from the peak of the continuum emission in both nuclei by
about 100 pc in the same direction. CO self-absorption is detected at the
centers of both nuclei but it is much deeper in the eastern nucleus. We also
clearly detect strong, highly redshifted CO absorption located near the
southwest side of each nucleus. For the eastern nucleus, we reproduce the major
line profile features with a simple kinematic model of a highly turbulent,
rotating disk with a substantial line center optical depth and a large gradient
in the excitation temperature. The red/blue asymmetries and line-to-continuum
offset are likely produced by absorption of the blue (SW) sides of the two
nuclei by blue-shifted, foreground molecular gas; the mass of the absorber is
comparable to the nuclear warm gas mass (10^8 M_solar). We measure an unusually
high L_CO/L_FIR ratio in the eastern nucleus, suggesting there is an additional
energy source, such as mechanical energy from shocks, present in this nucleus.
