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Inadequate turbulent support in low-metallicity molecular clouds

Nature Astronomy volume 9, pages 406–416 (2025)Cite this article

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Abstract

The dynamic properties of molecular clouds are set by the interplay of their self-gravity, turbulence, external pressure and magnetic fields. Extended surveys of Galactic molecular clouds typically find that their kinetic energy (Ek) counterbalances their self-gravitational energy (Eg), setting their virial parameter αvir = 2Ek/∣Eg∣ ≈ 1. However, past studies either have been biased by the use of optically thick lines or have been limited within the solar neighbourhood and the inner Galaxy (Galactocentric radius Rgc < Rgc,⊙ ≈ 8 kpc). Here we present sensitive mapping observations of optically thin 13CO lines towards molecular clouds in the low-metallicity Galactic outer disk (Rgc ~ 9–24 kpc). By combining archival data from the inner Galaxy and four nearby metal-poor dwarf galaxies, we reveal a systematic trend of αvir, which declines from supervirial dynamic states in metal-rich clouds to extremely subvirial dynamic states in metal-poor clouds. In these metal-poor environments, turbulence alone is insufficient to counterbalance the self-gravity of a cloud. A cloud-volumetric magnetic field may replace turbulence as the dominant cloud-supporting mechanism in low-metallicity conditions, for example, the outermost galactic disks, dwarf galaxies and galaxies in the early Universe, which would then inevitably impact the initial conditions for star formation in such environments.

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Fig. 1: Distribution of molecular clouds on the Galactic plane.
Fig. 2: Variations of the cloud velocity dispersion.
Fig. 3: Cloud virial parameter as a function of the gas-phase metallicity.

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Data availability

The following data and figures are available via figshare at https://doi.org/10.6084/m9.figshare.27282924 (ref. 105): (1) reduced data cubes of the new observations presented in this work (subfolder GMPMC_line_fitscubes), (2) 13CO spectra and \({N}_{{{\rm{H}}}_{2}}\) maps (subfolder Supp_Figures/NH2_13CO_spectrum), (3) infrared and radio images (subfolder Supp_Figures/IR_Radio_outer_disk_clouds) and (4) distance PDFs of the Galactic molecular clouds (subfolder Supp_Figures/Distance_PDFs). This work is based on observations carried out under Projects 031-17 and 102-22 with the IRAM 30 m telescope, Projects ADS/JAO.ALMA#2013.1.00652.S, ADS/JAO.ALMA#2015.1.00581.S, ADS/JAO.ALMA#2019.1.01641.S and ADS/JAO.ALMA#2021.2.00175.S with ALMA, and Project Lin_L_22B_1 with SMT. Source data are provided with this paper.

Code availability

Code for calculating the critical densities can be obtained from GitHub (https://github.com/ZhiyuZhang/critical_densities).

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Acknowledgements

This work is supported by the National Key Research & Development (R&D) Programme of China (Grant No. 2023YFA1608204). Z.-Y.Z., L. Lin, Yichen Sun and G.L. acknowledge the support of the National Natural Science Foundation of China (NSFC; Grant Nos. 12173016 and 12041305), science research grants from the China Manned Space Project (Grant Nos. CMS-CSST-2021-A08 and CMS-CSST-2021-A07) and the Programme for Innovative Talents, Entrepreneur in Jiangsu. This work also benefited from the International Space Science Institute (ISSI/ISSI-BJ) in Bern and Beijing, thanks to funding for the team ‘Chemical abundances in the ISM: the litmus test of stellar IMF variations in galaxies across cosmic time’ (PIs D.R. and Z.-Y.Z.). J. Wang gives thanks for the support of the NSFC (Grant No. 12173067) and the Guangxi Talent Programme (Highland of Innovation Talents). Y.G. is supported by the Strategic Priority Research Programme of the Chinese Academy of Sciences (Grant No. XDB0800301). Yan Sun is supported by the Youth Innovation Promotion Association, CAS (Grant No. 2022085), and the Light of West China Programme (Grant No. xbzg-zdsys-202212). T.G.B. acknowledges support from the Leading Innovation and Entrepreneurship Team of Zhejiang Province of China (Grant No. 2023R01008). D.R. thanks the Italian National Institute for Astrophysics for funding the project ‘An in-depth theoretical study of CNO element evolution in galaxies’ through Finanziamento della Ricerca Fondamentale, Theory Grant Fu. Ob. 1.05.12.06.08. D.L. is a New Cornerstone investigator. H.B.L. is supported by the National Science and Technology Council of Taiwan (Grant Nos. 111-2112-M-110-022-MY3 and 113-2112-M-110-022-MY3). K.Q. acknowledges support from the NSFC (Grant Nos. 12425304 and U1731237) and the National Key R&D Programme of China (Grant Nos. 2023YFA1608204 and 2022YFA1603100). C.-W.T. is supported by the NSFC (Grant No. 11988101). J. Wu gives thanks for support from the NSFC (Grant No. 12041302) and the Tianchi Talent Programme of Xinjiang Uygur Autonomous Region. S.F. acknowledges support from the NSFC (Grant No. 12373023), a starting grant at Xiamen University and the presidential excellence fund at Xiamen University (Grant No. 20720220024). This work is based on observations carried out under Projects 031-17 and 102-22 with the IRAM 30 m telescope, Projects ADS/JAO.ALMA#2013.1.00652.S, ADS/JAO.ALMA#2015.1.00581.S, ADS/JAO.ALMA#2019.1.01641.S and ADS/JAO.ALMA#2021.2.00175.S with ALMA, and Project Lin_L_22B_1 with SMT. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan) and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. The Heinrich Hertz SMT is operated by the Arizona Radio Observatory, which is part of Steward Observatory at the University of Arizona.

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Authors and Affiliations

  1. School of Astronomy and Space Science, Nanjing University, Nanjing, China

    Lingrui Lin, Zhi-Yu Zhang, Yong Shi, Yichen Sun, Keping Qiu & Gan Luo

  2. Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing, China

    Lingrui Lin, Zhi-Yu Zhang, Yong Shi, Yichen Sun, Keping Qiu & Gan Luo

  3. Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning, China

    Junzhi Wang

  4. Department of Physics, Section of Astrophysics, Astronomy and Mechanics, Aristotle University of Thessaloniki, Thessaloniki, Greece

    Padelis P. Papadopoulos

  5. Research Center for Astronomy, Academy of Athens, Athens, Greece

    Padelis P. Papadopoulos

  6. Max-Planck-Institut für Radioastronomie, Bonn, Germany

    Yan Gong

  7. Purple Mountain Observatory and Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, Nanjing, China

    Yan Gong & Yan Sun

  8. Research Center for Astronomical Computing, Zhejiang Laboratory, Hangzhou, China

    Thomas G. Bisbas

  9. INAF, Astrophysics and Space Science Observatory, Bologna, Italy

    Donatella Romano

  10. Department of Astronomy, Tsinghua University, Beijing, China

    Di Li

  11. National Astronomical Observatories, Chinese Academy of Sciences, Beijing, China

    Di Li, Chao-Wei Tsai & Jingwen Wu

  12. Department of Physics, National Sun Yat-Sen University, Kaohsiung City, Taiwan

    Hauyu Baobab Liu

  13. Center of Astronomy and Gravitation, National Taiwan Normal University, Taipei, Taiwan

    Hauyu Baobab Liu

  14. Cosmic Dawn Center (DAWN), Copenhagen, Denmark

    Lijie Liu

  15. Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark

    Lijie Liu

  16. DTU-Space, Technical University of Denmark, Lyngby, Denmark

    Lijie Liu

  17. Institut de radioastronomie millimétrique, Domaine Universitaire de Grenoble, Saint-Martin d’Hères, France

    Gan Luo

  18. Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Beijing, China

    Chao-Wei Tsai

  19. School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing, China

    Chao-Wei Tsai & Jingwen Wu

  20. Department of Astronomy, Xiamen University, Xiamen, China

    Siyi Feng

  21. Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai, China

    Bo Zhang

Authors
  1. Lingrui Lin
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  2. Zhi-Yu Zhang
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Contributions

L. Lin led the project, conducted the data reduction and analyses, and drafted proposals and the paper. Z.-Y.Z. initiated and supervised the whole project and improved the paper. J. Wang helped with the data analysis and validating tests. P.P.P. instructed the general virial analysis and outlined the larger theoretical picture. Yong Shi helped extend this work to low-metallicity dwarf galaxies. Y.G. helped improve the paper and was involved in discussions. Yan Sun provided the initial catalogue and helped with calculating the cloud distances. Yichen Sun, T.G.B. and D.R. helped with the abundance ratios. D.L., H.B.L. and K.Q. helped with discussions on magnetic fields. B.Z. helped with validating the distance measurements. L. Liu, G.L., C.-W.T., J. Wu and S.F. helped with discussions and in improving the paper. All authors reviewed the paper and were involved in discussions, telescope proposals and observations on which the raw data and the analyses were based.

Corresponding authors

Correspondence to Zhi-Yu Zhang or Junzhi Wang.

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Supplementary Tables 1–3 and Figs. 1–11.

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Lin, L., Zhang, ZY., Wang, J. et al. Inadequate turbulent support in low-metallicity molecular clouds. Nat Astron 9, 406–416 (2025). https://doi.org/10.1038/s41550-024-02440-3

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