Resolved Structure of Arp 220 Nuclei at \lambda ~3 mm
  We analyze 3 mm emission of the ultraluminous infrared galaxy Arp 220 for
spatially-resolved structure and spectral properties of the merger nuclei. ALMA
archival data at ~0.05" resolution are used for extensive visibility fitting
and deep imaging of continuum emission. The data are fitted well with two
concentric components for each nucleus, such as two Gaussians or one Gaussian
plus one exponential disk. The larger components in individual nuclei are
similar in shape and extent, ~100-150 pc, to the cm-wave emission due to
supernovae. They are therefore identified with the known starburst nuclear
disks. The smaller components in both nuclei have about a few 10 pc sizes and
peak brightness temperatures (Tb) more than twice higher than in previous
single-Gaussian fitting. They correspond to the dust emission that we find
centrally concentrated in both nuclei by subtracting the plasma emission
measured at 33 GHz. The dust emission in the western nucleus is found to have a
peak Tb ~ 530 K and a full width at half maximum of about 20 pc. This component
is estimated to have a bolometric luminosity on the order of 10^{12.5} Lsun and
a 20 pc-scale luminosity surface density 10^{15.5} Lsun/kpc^2. A luminous AGN
is a plausible energy source for these high values while other explanations
remain to be explored. Our continuum image also reveals a third structural
component of the western nucleus --- a pair of faint spurs perpendicular to the
disk major axis. We attribute it to a bipolar outflow from the highly inclined
(i ~ 60 deg) western nuclear disk.
