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

Juan Li, Junzhi Wang, Zhiqiang Shen, Alba Vidal-Garcia, Yuqiang Li, DI Li, Liubin Pan, Lei Huang, Fengyao Zhu, Siqi Zheng, Yiping Ao, Alvaro Sanchez-Momge, Zhiyu Zhang, Xing Lu, Tie Liu, Xingwu Zheng

TL;DR

This work maps NH3 metastable lines (3,3)–(18,18) toward G0.66-0.13 in the Galactic Center to characterize hot molecular gas in the CMZ. The detection of NH3 (17,17) and (18,18) in emission, with upper energy levels up to 3100 K, reveals a widespread hot gas component (>400 K) that comprises about 5% of the NH3. Rotational-diagram analysis shows two temperature components (warm ~120–140 K and hot >400 K) and indicates distinct spatial distributions, with hot gas concentrated at interfaces between warm clouds. The authors argue that intermittency turbulence heating, driven by shear between clouds in the Galactic potential, can reproduce the complex CMZ temperature structure and may be a widespread mechanism in galactic nuclei, underscoring the role of shear-induced turbulence in galactic evolution.

Abstract

We observed NH3 metastable inversion lines from (3, 3) to (18, 18) toward G0.66-0.13 in the Galactic center with the Shanghai Tianma 65m radio telescope and Yebes 40 m telescope. Highly-excited lines of NH3 (17, 17), (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 NH3 (13, 13) toward G0.66-0.13. The rotation temperatures of hot gas traced by NH3 exceed 400 K, which amounts to five percent of the total NH3 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, especially the hot gas observed here. The results presented here demonstrate that turbulence heating dominates the heating of the molecular gas in the Central Molecular Zone, 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.

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

TL;DR

This work maps NH3 metastable lines (3,3)–(18,18) toward G0.66-0.13 in the Galactic Center to characterize hot molecular gas in the CMZ. The detection of NH3 (17,17) and (18,18) in emission, with upper energy levels up to 3100 K, reveals a widespread hot gas component (>400 K) that comprises about 5% of the NH3. Rotational-diagram analysis shows two temperature components (warm ~120–140 K and hot >400 K) and indicates distinct spatial distributions, with hot gas concentrated at interfaces between warm clouds. The authors argue that intermittency turbulence heating, driven by shear between clouds in the Galactic potential, can reproduce the complex CMZ temperature structure and may be a widespread mechanism in galactic nuclei, underscoring the role of shear-induced turbulence in galactic evolution.

Abstract

We observed NH3 metastable inversion lines from (3, 3) to (18, 18) toward G0.66-0.13 in the Galactic center with the Shanghai Tianma 65m radio telescope and Yebes 40 m telescope. Highly-excited lines of NH3 (17, 17), (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 NH3 (13, 13) toward G0.66-0.13. The rotation temperatures of hot gas traced by NH3 exceed 400 K, which amounts to five percent of the total NH3 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, especially the hot gas observed here. The results presented here demonstrate that turbulence heating dominates the heating of the molecular gas in the Central Molecular Zone, 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.
Paper Structure (14 sections, 4 equations)

This paper contains 14 sections, 4 equations.

Table of Contents

  1. Introduction
  2. Observations and data reduction
  3. Yebes 40m observations
  4. TMRT K band observations
  5. TMRT Ka band observations
  6. Data reductions
  7. Results
  8. Detection of NH$_3$ highly-excited metastable inversion lines
  9. Spatial distribution of NH$_3$ (6, 6) and (13, 13)
  10. Estimate of physical conditions
  11. Discussion
  12. Line cooling timescale of hot molecular gas
  13. Heating mechanism of hot molecular gas
  14. Conclusions