NH3 in the Galactic Center is formed in Cool Conditions

TL;DR

This study reveals that NH_3 molecules in the Galactic Center are formed on cold dust grains at 11-20 K and released into the gas phase by shocks, explaining the temperature discrepancy between gas and dust.

Contribution

It provides observational evidence that NH_3 forms on cold dust grains in the Galactic Center, linking molecular formation conditions to dust temperature and shock processes.

Findings

NH_3 ortho-to-para ratio indicates formation at 11-20 K.

NH_3 formation occurs on dust grains at cold temperatures.

Transient heating explains the temperature difference between gas and dust.

Abstract

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.