ALMA Imaging of HCN, CS and dust in Arp 220 and NGC 6240
  We report ALMA Band 7 (350 GHz) imaging at 0.4 - 0.6arcsec resolution and
Band 9 (696 GHz) at ~0.25arcsec resolution of the luminous IR galaxies Arp 220
and NGC 6240. The long wavelength dust continuum is used to estimate ISM masses
for Arp 220 East, West and NGC 6240 of 1.9, 4.2 and 1.6x10^9 msun within radii
of 69, 65 and 190 pc. The HCN emission was modeled to derive the emissivity
distribution as a function of radius and the kinematics of each nuclear disk,
yielding dynamical masses consistent with the masses and sizes derived from the
dust emission. In Arp 220, the major dust and gas concentrations are at radii
less than 50 pc in both counter-rotating nuclear disks. The thickness of the
disks in Arp 220estimated from the velocity dispersion and rotation velocities
are 10-20 pc and the mean gas densities are n_H2 ~10^5 cm^-3 at R < 50 pc. We
develop an analytic treatment for the molecular excitation (including photon
trapping), yielding volume densities for both the HCN and CS emission with n_H2
~2x10^5 cm^-3. The agreement of the mean density from the total mass and size
with that required for excitation suggests that the volume is essentially
filled with dense gas, i.e. it is not cloudy or like swiss cheese.
