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

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: yyan@mpifr-bonn.mpg.de; astrotingyan@gmail.com
² Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 830011 Urumqi, PR China
³ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA
⁴ School of Astronomy & Space Science, Nanjing University, 163 Xianlin Avenue, Nanjing 210023, PR China

Received: 13 December 2023
Accepted: 23 March 2024

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.