ALMA Resolves the Nuclear Disks of Arp 220
  We present 90 mas (37 pc) resolution ALMA imaging of Arp 220 in the CO (1-0)
line and continuum at $\lambda = 2.6$ mm. The internal gas distribution and
kinematics of both galactic nuclei are well-resolved for the first time. In the
West nucleus, the major gas and dust emission extends out to 0.2\arcsec radius
(74 pc); the central resolution element shows a strong peak in the dust
emission but a factor 3 dip in the CO line emission. In this nucleus, the dust
is apparently optically thick ($\tau_{\rm 2.6mm} \sim1$) at $\lambda = 2.6$ mm
with a dust brightness temperature $\sim147$ K. The column of ISM at this
nucleus is $\rm N_{H2} \geq 2\times10^{26}$ cm$^{-2}$, corresponding to
$\sim$900 gr cm$^{-2}$. The East nucleus is more elongated with radial extent
0.3\arcsec or $\sim111$ pc. The derived kinematics of the nuclear disks provide
a good fit to the line profiles, yielding the emissivity distributions, the
rotation curves and velocity dispersions. In the West nucleus, there is
evidence of a central Keplerian component requiring a central mass of
$8\times10^8$ \msun. The intrinsic widths of the emission lines are $\Delta \rm
v (FWHM)$ = 250 (West) and 120 (East) \kms. Given the very short dissipation
timescales for turbulence ($\lesssim10^5$ yrs), we suggest that the line widths
may be due to semi-coherent motions within the nuclear disks. The symmetry of
the nuclear disk structures is impressive -- implying the merger timescale is
significantly longer than the rotation period of the disks.
