Ammonia as a Temperature Tracer in the Ultraluminous Galaxy Merger
  Arp220
  (abridged) We present ATCA and GBT observations of ammonia (NH3) toward the
ultraluminous infrared galaxy (ULIRG) merger Arp220. We detect the NH3 (1,1),
(2,2), (3,3), (4,4), (5,5), and (6,6) inversion lines in absorption against the
unresolved, (62+/-9)mJy continuum source at 1.2cm. The peak apparent optical
depths of the NH3 lines range from ~0.05 to 0.18. The absorption depth of the
NH3 (1,1) line is significantly shallower than expected based on the depths of
the other transitions, which might be caused by contamination from emission by
a hypothetical, cold (<~20K) gas layer with an estimated column density of <~
2x10^14 cm^-2. The widths of the NH3 absorption lines are ~120-430 km s^-1, in
agreement with those of other molecular tracers. We cannot confirm the
extremely large linewidths of up to ~1800km s^-1 previously reported. We
determine a rotational temperature of (124+/-19)K, corresponding to a kinetic
temperature of T_kin=(186+/-55)K. NH3 column densities depend on the excitation
temperature. For an excitation temperature of 50K, we estimate
(8.4+/-0.5)x10^16cm^-2. The relation scales linearly for possible higher
excitation temperatures. In the context of a model with a molecular ring that
connects the two nuclei in Arp220, we estimate the H2 gas density to be
~f_V^-0.5 x (1-4)x10^3, (f_V: volume filling factor). In addition to NH3, our
ATCA data show an absorption feature adjacent in frequency to the NH3 (3,3)
line. If we interpret the line to be from the OH ^2Pi_3/2 J=9/2 F=4-4
transition, it would have a linewidth, systemic velocity, and apparent optical
depth similar to what we detect in the NH3 lines. If this association with OH
is correct, it marks the first detection of the highly excited (~511K above
ground state) ^2Pi_3/2 J=9/2 F=4-4 OH line in an extragalactic object.
