NH3 in the Galactic center is formed under cool conditions

Abstract

It is an open question as to 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 in the 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 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 transient heating of the interstellar media in the CMZ approximately 105 yr ago when NH3 molecules were released from the dust. © 2009. Astronomical Society of Japan.