2013ApJ...772..105M

We present the detection of metastable inversion lines of ammonia (NH₃) from energy levels high above the ground state. We detect these lines in both emission and absorption toward 15 of 17 positions in the central 300 pc of the Galaxy. In total, we observe seven metastable transitions of NH₃: (8, 8), (9, 9), (10, 10), (11, 11), (12, 12), (13, 13) and (15, 15), with energies (in Kelvins) ranging from 680 to 2200 K. We also mapped emission from NH₃(8, 8) and (9, 9) in two clouds in the Sgr A complex (M-0.02-0.07 and M-0.13-0.08), and we find that the line emission is concentrated toward the dense centers of these molecular clouds. The rotational temperatures derived from the metastable lines toward M-0.02-0.07 and M-0.13-0.08 and an additional cloud (M0.25+0.01) range from 350 to 450 K. Similarly highly-excited lines of NH₃ have previously been observed toward Sgr B2, where gas with kinetic temperatures of ∼600 K had been inferred. Our observations show that the existence of a hot molecular gas component is not unique to Sgr B2, but rather appears common to many Galactic center molecular clouds. In M-0.02-0.07, we find that the hot NH₃ contributes ∼10% of the cloud's total NH₃column density, and further, that the hot NH₃ in this cloud arises in gas which is extended or uniformly distributed on ≳ 10 arcsec scales. We discuss the implications of these constraints upon the nature of this hot gas component. In addition to the detection of hot metastable NH₃ line emission, we also detect for the first time emission from nonmetastable inversion transitions of NH₃ in both M-0.02-0.07 and M-0.13-0.08.