NH3 in the Galactic Center is formed in Cool Conditions
  It is an open question why the temperature of molecular gas in the Galactic
center region is higher than that of dust. To address this problem, we made
simultaneous observations in the NH_3 (J,K) = (1,1), (2,2), and (3,3) lines of
the central molecular zone (CMZ) using the Kagoshima 6 m telescope. The
ortho-to-para ratio of NH_3 molecules in the CMZ is 1.5--3.5 at most observed
area. This ratio is higher than the statistical equilibrium value, and suggests
that the formation temperature of NH_3 is 11--20 K. This temperature is similar
to the dust temperature estimated from the submillimeter and infrared
continuum. This result suggests that the NH_3 molecules in the CMZ were
produced on dust grains with the currently observed temperature (11--20 K), and
they were released into the gas phase by supernova shocks or collisions of dust
particles. The discrepancy between warm molecular gas and cold dust can be
explained by the transient heating of the interstellar media in the CMZ
approximately 10^5 years ago when the NH_3 molecules were released from the
dust.
