NH3 in the Galactic Center is Formed under 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 NH3 (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 NH3 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 NH3 is 11–20 K. This temperature is similar to the dust temperature estimated from the submillimeter and infrared continuum. This result suggests that the NH3 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 NH3 molecules were released from the dust.