Widespread Hot Molecular Gas Heated by Shear-induced Turbulence in the Galactic Center

Abstract We observed NH 3 metastable inversion lines from (3, 3) to (18, 18) toward G0.66-0.13 in the Galactic center with the Shanghai Tianma 65 m radio telescope and Yebes 40 m telescope. Highly excited lines of NH 3 (17, 17) and (18, 18) were detected in emission for the first time in the interstellar medium, with upper energy levels up to 3100 K. Mapping observations reveal widespread hot molecular gas traced by NH 3 (13, 13) toward G0.66-0.13. The rotation temperatures of hot gas traced by NH 3 exceed 400 K, which amounts to 5% of the total NH 3 in the Galactic center. Hot gas (>400 K) and warm gas (100–140 K) are found in distinct clumps, with the hot gas located at the interfacing regions between different warm clouds. The theory of intermittency in turbulence reproduces the complex temperature structure in the central molecular zone (CMZ), especially the hot gas observed here. The results presented here demonstrate that turbulence heating dominates the heating of the molecular gas in the CMZ, while the turbulence is induced by the shear motion of molecular clouds under the gravitational potential of the nuclear star clusters and the supermassive black hole. Our results suggest that shear-induced turbulence heating could be a widespread factor influencing galactic evolution.