Discovery of widespread non-metastable ammonia masers in the Milky Way

Abstract

We present the results of a search for ammonia maser emission in 119 Galactic high-mass star-forming regions (HMSFRs) known to host 22 GHz H2O maser emission. Our survey has led to the discovery ofnon-metastable NH3 inversion line masers toward 14 of these sources. This doubles the number of known non-metastable ammonia masers in our Galaxy, including nine new very high-excitation (J, K) = (9,6) maser sources. These maser lines, including NH3 (5,4), (6,4), (6,5), (7,6), (8,6), (9,6), (9,8), (10,8), and (11,9), arise from energy levels of 342 K, 513 K, 465 K, 606 K, 834 K, 1090 K, 942 K, 1226 K, and 1449 K above the ground state. Additionally, we tentatively report a new metastable NH3 (3,3) maser in G048.49 and an NH3 (7,7) maser in G029.95. Our observations reveal that all of the newly detected NH3 maser lines exhibit either blueshifted or redshifted velocities with respect to the source systemic velocities. Among the non-metastable ammonia maser lines, larger velocity distributions, offset from the source systemic velocities, are found in the ortho-NH3 (K = 3n) transitions than in the para-NH3 (K ≠ 3n) ones.