Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Article
  • Published:

A network of velocity-coherent filaments formed by supersonic turbulence in a very-high-velocity H i cloud

Nature Astronomy volume 9, pages 1366–1374 (2025)Cite this article

Subjects

Abstract

The warm neutral medium (WNM) was thought to be subsonically/transonically turbulent, and it lacks a network of intertwined filaments that are commonly seen in both molecular clouds and the cold neutral medium. We report H i 21-cm-line observations of a very-high-velocity (−330 km s−1 < VLSR < −250 km s−1) cloud (VHVC), using the Five-Hundred-Meter Aperture Spherical Radio Telescope (FAST), with very high resolution and sensitivity. Such a VHVC is here clearly revealed to be a supersonic WNM system consisting of a network of velocity-coherent H i filaments. The filaments are in the forms of slim curves, hubs and webs, distributed in different layers within the position–position–velocity (ppv) data cube. The entire cloud has skewed lognormal probability distribution of column density and the filaments themselves show asymmetrical radial density profiles, indicating shock compression by supersonic magnetohydrodynamic (MHD) turbulence, as is also confirmed by our MHD simulation (sonic Mach number Ms = 3 and Alfvén Mach number MA = 1). This suggests that hierarchical filaments can be established by shocks in a low-density WNM, where gravity is negligible, offering a viable pathway to structure formation in the earliest evolutionary phases of the interstellar medium.

This is a preview of subscription content, access via your institution

Access options

Buy this article

  • Purchase on SpringerLink
  • Instant access to the full article PDF.

USD 39.95

Prices may be subject to local taxes which are calculated during checkout

Fig. 1: Spatial distribution and example spectra of the H i 21-cm lines in the G165 VHVC.
Fig. 2: H i distribution of G165 VHVC in ppv space.
Fig. 3: Statistical properties of the H i layers and filaments in the G165 VHVC.

Similar content being viewed by others

Data availability

The FAST H i cube of the G165 VHVC, smoothed to a velocity resolution of ~1 km s−1, can be found at https://zenodo.org/records/15347725. Owing to data size, the full data used for this study are available from the corresponding authors upon reasonable request. Source data are provided with this paper.

Code availability

The FAST OTF data reduction script we developed for this work can found at https://gitee.com/liuxunchuan/fast_otf_datareduct. The filament-extracting code we developed for this work, ClimbRidge and fil3d, can be found at https://gitee.com/liuxunchuan/sometools/tree/master/filament/. The MST analysis tool is available at https://pypi.org/project/mistree/. The compared filament-extracting method FilFinder can be found at https://github.com/e-koch/FilFinder. Image processing package scikit-image can be found at https://pypi.org/project/scikit-image/. The ORION2 code is available at https://joss.theoj.org/papers/10.21105/joss.03771.

References

  1. Kalberla, P. M. W. & Kerp, J. The H i distribution of the Milky Way. Annu. Rev. Astron. Astrophys. 47, 27–61 (2009).

    Article  ADS  Google Scholar 

  2. McClure-Griffiths, N. M., Stanimirović, S. & Rybarczyk, D. R. Atomic hydrogen in the Milky Way: a stepping stone in the evolution of galaxies. Annu. Rev. Astron. Astrophys. 61, 19–63 (2023).

    Article  ADS  Google Scholar 

  3. André, P. et al. From filamentary clouds to prestellar cores to the stellar IMF: initial highlights from the Herschel Gould Belt Survey. Astron. Astrophys. 518, L102 (2010).

    Article  ADS  Google Scholar 

  4. Könyves, V. et al. A census of dense cores in the Aquila cloud complex: SPIRE/PACS observations from the Herschel Gould Belt survey. Astron. Astrophys. 584, A91 (2015).

    Article  Google Scholar 

  5. Li, G.-X. et al. ATLASGAL: a Galaxy-wide sample of dense filamentary structures. Astron. Astrophys. 591, A5 (2016).

    Article  ADS  Google Scholar 

  6. Liu, T. et al. The TOP-SCOPE Survey of Planck Galactic Cold Clumps: survey overview and results of an exemplar source, PGCC G26.53+0.17. Astrophys. J. Suppl. Ser. 234, 28 (2018).

    Article  ADS  Google Scholar 

  7. Liu, X. C. et al. A search for cloud cores affected by shocked carbon chain chemistry in L1251. Astrophys. J. 912, 148 (2021).

    Article  ADS  Google Scholar 

  8. Zhou, J.-W. et al. ATOMS: ALMA Three-millimeter Observations of Massive Star-forming regions—XI. From inflow to infall in hub–filament systems. Mon. Not. R. Astron. Soc. 514, 6038–6052 (2022).

    Article  ADS  Google Scholar 

  9. Hacar, A. et al. Initial conditions for star formation: a physical description of the filamentary ISM. In Protostars and Planets VII Conference Series Vol. 534 (eds Inutsuka, S. et al.) 153 (Astronomical Society of the Pacific, 2023).

  10. Liu, X. et al. The ALMA-QUARKS Survey. I. Survey description and data reduction. Res. Astron. Astrophys. 24, 025009 (2024).

    Article  ADS  Google Scholar 

  11. Clark, S. E., Peek, J. E. G. & Putman, M. E. Magnetically aligned H i fibers and the rolling Hough transform. Astrophys. J. 789, 82 (2014).

    Article  ADS  Google Scholar 

  12. Kalberla, P. M. W. et al. Cold Milky Way H i gas in filaments. Astrophys. J. 821, 117 (2016).

    Article  ADS  Google Scholar 

  13. Kim, D. A., Clark, S. E., Putman, M. E. & Li, L. The kinematic structure of magnetically aligned H i filaments. Mon. Not. R. Astron. Soc. 526, 4345–4358 (2023).

    Article  ADS  Google Scholar 

  14. Xu, S., Ji, S. & Lazarian, A. On the formation of density filaments in the turbulent interstellar medium. Astrophys. J. 878, 157 (2019).

    Article  ADS  Google Scholar 

  15. Muller, C. A., Oort, J. H. & Raimond, E. Hydrogène neutre dans la couronne galactique? C. R. Acad. Sci. 257, 1661–1662 (1963).

    Google Scholar 

  16. Wakker, B. P. Distribution and origin of high-velocity clouds. II. Statistical analysis of the whole-sky survey. Astron. Astrophys. 250, 499–508 (1991).

    ADS  Google Scholar 

  17. van Woerden, H., Schwarz, U. J., Peletier, R. F., Wakker, B. P. & Kalberla, P. M. W. A confirmed location in the Galactic halo for the high-velocity cloud ‘chain A’. Nature 400, 138–141 (1999).

    Article  ADS  Google Scholar 

  18. Wakker, B. P. et al. Distances to galactic high-velocity clouds. I. Cohen Stream, Complex GCP, cloud g1. Astrophys. J. 672, 298–319 (2008).

    Article  ADS  Google Scholar 

  19. Thom, C. et al. An accurate distance to high-velocity cloud Complex C. Astrophys. J. 684, 364–372 (2008).

    Article  ADS  Google Scholar 

  20. Peek, J. E. G. et al. The first distance constraint on the renegade high-velocity cloud Complex WD. Astrophys. J. Lett. 828, L20 (2016).

    Article  ADS  Google Scholar 

  21. Westmeier, T. A new all-sky map of Galactic high-velocity clouds from the 21-cm HI4PI survey. Mon. Not. R. Astron. Soc. 474, 289–299 (2018).

    Article  ADS  Google Scholar 

  22. Bregman, J. N. The galactic fountain of high-velocity clouds. Astrophys. J. 236, 577–591 (1980).

    Article  ADS  Google Scholar 

  23. de Boer, K. S. The contribution of halo red giant mass loss to the high-velocity gas falling onto the Milky Way disk. Astron. Astrophys. 419, 527–531 (2004).

    Article  ADS  Google Scholar 

  24. Fox, A. J. et al. On the metallicity and origin of the Smith High-Velocity Cloud. Astrophys. J. Lett. 816, L11 (2016).

    Article  ADS  Google Scholar 

  25. Cashman, F. H. et al. Caught in the act: a metal-rich high-velocity cloud in the Inner Galaxy. Astrophys. J. 944, 65 (2023).

    Article  ADS  Google Scholar 

  26. Peek, J. E. G. et al. The GALFA-H i Survey Data Release 2. Astrophys. J. Suppl. Ser. 234, 2 (2017).

    Article  ADS  Google Scholar 

  27. Martin, P. G. et al. GHIGLS: H i mapping at intermediate Galactic latitude using the Green Bank Telescope. Astrophys. J. 809, 153 (2015).

    Article  ADS  Google Scholar 

  28. Blagrave, K. et al. DHIGLS: DRAO H i Intermediate Galactic Latitude Survey. Astrophys. J. 834, 126 (2017).

    Article  ADS  Google Scholar 

  29. Lockman, F. J., Benjamin, R. A., Heroux, A. J. & Langston, G. I. The Smith Cloud: a high-velocity cloud colliding with the Milky Way. Astrophys. J. Lett. 679, L21 (2008).

    Article  ADS  Google Scholar 

  30. Holm-Hansen, C., Putman, M. E. & Kim, D. A. Properties of 3D H I filaments in the Smith High Velocity Cloud. Mon. Not. R. Astron. Soc. 536, 3507–3517 (2025).

    Article  ADS  Google Scholar 

  31. Marchal, A., Martin, P. G. & Gong, M. Resolving the formation of cold H i filaments in the high-velocity cloud Complex C. Astrophys. J. 921, 11 (2021).

    Article  ADS  Google Scholar 

  32. Vujeva, L., Marchal, A., Martin, P. G. & Taank, M. Mapping the multiphase structure of H i in the low-latitude intermediate-velocity Arch 1. Astrophys. J. 951, 120 (2023).

    Article  ADS  Google Scholar 

  33. Wakker, B. P. & van Woerden, H. Distribution and origin of high-velocity clouds. III. Clouds, complexes and populations. Astron. Astrophys. 250, 509–532 (1991).

    ADS  Google Scholar 

  34. Giovanelli, R. Very high-velocity H i clouds—an intergalactic population. Astron. J. 86, 1468–1479 (1981).

    Article  ADS  Google Scholar 

  35. Hulsbosch, A. N. M. A high sensitivity survey of hydrogen at very high velocities (preliminary report). Astron. Astrophys. 66, L5–L8 (1978).

    ADS  Google Scholar 

  36. Giovanelli, R. Studies of high-velocity clouds. I. A high-sensitivity survey. Astron. J. 85, 1155–1181 (1980).

    Article  ADS  Google Scholar 

  37. McClure-Griffiths, N. M. et al. GASS: the Parkes Galactic All-Sky Survey. I. Survey description, goals, and initial data release. Astrophys. J. Suppl. Ser. 181, 398–412 (2009).

    Article  ADS  Google Scholar 

  38. Kerp, J., Winkel, B., Ben Bekhti, N., Flöer, L. & Kalberla, P. M. W. The Effelsberg Bonn H i Survey (EBHIS). Astron. Nachr. 332, 637–648 (2011).

    Article  ADS  Google Scholar 

  39. Winkel, B. et al. The Effelsberg–Bonn H i Survey: Milky Way gas. First data release. Astron. Astrophys. 585, A41 (2016).

    Article  Google Scholar 

  40. HI4PI Collaboration HI4PI: a full-sky H i survey based on EBHIS and GASS. Astron. Astrophys. 594, A116 (2016).

    Article  Google Scholar 

  41. Wannier, P., Wrixon, G. T. & Wilson, R. W. A survey of positive velocity neutral hydrogen above the Galactic Plane between l = 252° and l = 322°. Astron. Astrophys. 18, 224–231 (1972).

    ADS  Google Scholar 

  42. Mathewson, D. S., Cleary, M. N. & Murray, J. D. The Magellanic Stream. Astrophys. J. 190, 291–296 (1974).

    Article  ADS  Google Scholar 

  43. Bajaja, E. et al. A new general survey of high-velocity neutral hydrogen in the southern hemisphere. Astrophys. J. Suppl. Ser. 58, 143–165 (1985).

    Article  ADS  Google Scholar 

  44. Hulsbosch, A. N. M. & Wakker, B. P. A deep, nearly complete, survey of northern high-velocity clouds. Astron. Astrophys. Suppl. 75, 191–236 (1988).

    ADS  Google Scholar 

  45. Henshaw, J. D. et al. Ubiquitous velocity fluctuations throughout the molecular interstellar medium. Nat. Astron. 4, 1064–1071 (2020).

    Article  ADS  Google Scholar 

  46. Heiles, C. & Troland, T. H. The Millennium Arecibo 21 Centimeter Absorption-Line Survey. II. Properties of the warm and cold neutral media. Astrophys. J. 586, 1067–1093 (2003).

    Article  ADS  Google Scholar 

  47. Wolfire, M. G., McKee, C. F., Hollenbach, D. & Tielens, A. G. G. M. Neutral atomic phases of the interstellar medium in the Galaxy. Astrophys. J. 587, 278–311 (2003).

    Article  ADS  Google Scholar 

  48. Padoan, P., Nordlund, A. & Jones, B. J. T. The universality of the stellar initial mass function. Mon. Not. R. Astron. Soc. 288, 145–152 (1997).

    Article  ADS  Google Scholar 

  49. Burkhart, B. & Lazarian, A. The column density variance–Ms relationship. Astrophys. J. Lett. 755, L19 (2012).

    Article  ADS  Google Scholar 

  50. Burkhart, B., Lee, M.-Y., Murray, C. E. & Stanimirović, S. The lognormal probability distribution function of the Perseus molecular cloud: a comparison of H i and dust. Astrophys. J. Lett. 811, L28 (2015).

    Article  ADS  Google Scholar 

  51. Federrath, C., Roman-Duval, J., Klessen, R. S., Schmidt, W. & Mac Low, M. M. Comparing the statistics of interstellar turbulence in simulations and observations. Solenoidal versus compressive turbulence forcing. Astron. Astrophys. 512, A81 (2010).

    Article  ADS  Google Scholar 

  52. Squire, J. & Hopkins, P. F. The distribution of density in supersonic turbulence. Mon. Not. R. Astron. Soc. 471, 3753–3767 (2017).

    Article  ADS  Google Scholar 

  53. Haud, U. & Kalberla, P. M. W. Gaussian decomposition of H i surveys. III. Local H i. Astron. Astrophys. 466, 555–564 (2007).

    Article  ADS  Google Scholar 

  54. Chen, Y.-C., Ho, S., Freeman, P. E., Genovese, C. R. & Wasserman, L. Cosmic web reconstruction through density ridges: method and algorithm. Mon. Not. R. Astron. Soc. 454, 1140–1156 (2015).

    Article  ADS  Google Scholar 

  55. Chen, Y.-C. et al. Cosmic web reconstruction through density ridges: catalogue. Mon. Not. R. Astron. Soc. 461, 3896–3909 (2016).

    Article  ADS  Google Scholar 

  56. Audit, E. & Hennebelle, P. Thermal condensation in a turbulent atomic hydrogen flow. Astron. Astrophys. 433, 1–13 (2005).

    Article  ADS  Google Scholar 

  57. Palmeirim, P. et al. Herschel view of the Taurus B211/3 filament and striations: evidence of filamentary growth? Astron. Astrophys. 550, A38 (2013).

    Article  Google Scholar 

  58. Peretto, N. et al. Global collapse of molecular clouds as a formation mechanism for the most massive stars. Astron. Astrophys. 555, A112 (2013).

    Article  Google Scholar 

  59. Fernández-López, M. et al. CARMA Large Area Star Formation Survey: observational analysis of filaments in the Serpens South molecular cloud. Astrophys. J. Lett. 790, L19 (2014).

    Article  ADS  Google Scholar 

  60. Nan, R. et al. The Five-hundred-meter Aperture Spherical radio Telescope (FAST) Project. Int. J. Mod. Phys. D 20, 989–1024 (2011).

    Article  ADS  Google Scholar 

  61. Jiang, P. et al. Commissioning progress of the FAST. Sci. China Phys. Mech. Astron. 62, 959502 (2019).

    Article  ADS  Google Scholar 

  62. Liu, X. et al. A FAST survey of H i narrow-line self-absorptions in Planck Galactic cold clumps guided by HC3N. Astron. Astrophys. 658, A140 (2022).

    Article  ADS  Google Scholar 

  63. Liu, X. FAST_otf_datareduct. Gitee https://gitee.com/liuxunchuan/fast_otf_datareduct (2023).

  64. Liu, X. et al. FAST H i data of G165 VHVC. Zenodo https://doi.org/10.5281/zenodo.15347725 (2025).

  65. Condon, J. J. et al. The NRAO VLA Sky Survey. Astron. J. 115, 1693–1716 (1998).

    Article  ADS  Google Scholar 

  66. Henshaw, J. D. et al. Molecular gas kinematics within the central 250 pc of the Milky Way. Mon. Not. R. Astron. Soc. 457, 2675–2702 (2016).

    Article  ADS  Google Scholar 

  67. Henshaw, J. D. et al. ‘The Brick’ is not a brick: a comprehensive study of the structure and dynamics of the central molecular zone cloud G0.253+0.016. Mon. Not. R. Astron. Soc. 485, 2457–2485 (2019).

    Article  ADS  Google Scholar 

  68. Liu, X. ClimbRidge. Gitee https://gitee.com/liuxunchuan/sometools/tree/master/filament/ClimbRidge (2025).

  69. Koch, E. W. & Rosolowsky, E. W. Filament identification through mathematical morphology. Mon. Not. R. Astron. Soc. 452, 3435–3450 (2015).

    Article  ADS  Google Scholar 

  70. Liu, X. Fil3d. Gitee https://gitee.com/liuxunchuan/sometools/tree/master/filament/Fil3d (2025).

  71. Naidoo, K. mistree 2.0.0. PyPI http://pypi.org/project/mistree/ (2024).

  72. Li, D. & Goldsmith, P. F. H i narrow self-absorption in dark clouds. Astrophys. J. 585, 823–839 (2003).

    Article  ADS  Google Scholar 

  73. Li, P. S., Martin, D. F., Klein, R. I. & McKee, C. F. A stable, accurate methodology for high Mach number, strong magnetic field MHD turbulence with adaptive mesh refinement: resolution and refinement studies. Astrophys. J. 745, 139 (2012).

    Article  ADS  Google Scholar 

  74. Li, P. et al. ORION2: a magnetohydrodynamics code for star formation. J. Open Source Softw. 6, 3771 (2021).

    Article  ADS  Google Scholar 

  75. Colella, P. Multidimensional upwind methods for hyperbolic conservation laws. J. Comput. Phys. 87, 171–200 (1990).

    Article  ADS  MathSciNet  Google Scholar 

  76. Evans, C. R. & Hawley, J. F. Simulation of magnetohydrodynamic flows: a constrained transport model. Astrophys. J. 332, 659–677 (1988).

    Article  ADS  Google Scholar 

  77. Stone, E. C. et al. An asymmetric solar wind termination shock. Nature 454, 71–74 (2008).

    Article  ADS  Google Scholar 

  78. Mac Low, M.-M. The energy dissipation rate of supersonic, magnetohydrodynamic turbulence in molecular clouds. Astrophys. J. 524, 169–178 (1999).

    Article  ADS  Google Scholar 

Download references

Acknowledgements

We wish to thank the staff of FAST for their help provided during the observations. X. Liu and T.L. have been supported by the National Key R&D Program of China (2022YFA1603100). X. Liu has also been supported by the Strategic Priority Research Program of the Chinese Academy of Sciences under grant XDB0800303. T.L. acknowledges the National Natural Science Foundation of China (NSFC) 12017061 and 12122307. T.L. also acknowledges support by the Tianchi Talent Program of Xinjiang Uygur Autonomous Region and the PIFI program of Chinese Academy of Sciences through grant 2025PG0009. P.-S.L. acknowledges NSFC 1241101426. S.-L.Q. acknowledges NSFC No.12033005. N.T. acknowledges support by the NSFC 12473023 and by the University Annual Scientific Research Plan of Anhui Province (2023AH030052 and 2022AH010013). This research was carried out in part at the Jet Propulsion Laboratory, which is operated by the California Institute of Technology under a contract with the National Aeronautics and Space Administration (80NM0018D0004).

Author information

Authors and Affiliations

  1. Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai, People’s Republic of China

    Xunchuan Liu, Tie Liu, Pak-Shing Li, Xiaofeng Mai, Xing Lu, Qiuyi Luo, Yihuan Di, Qilao Gu & Zhiqiang Shen

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

    Christian Henkel

  3. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

    Paul F. Goldsmith

  4. School of Physics and Astronomy, Yunnan University, Kunming, People’s Republic of China

    Sheng-Li Qin & Hong-Li Liu

  5. Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, People’s Republic of China

    Yan Gong

  6. Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing, People’s Republic of China

    Fengwei Xu

  7. Department of Astronomy, School of Physics, Peking University, Beijing, People’s Republic of China

    Fengwei Xu & Yuefang Wu

  8. Research Center for Astronomical Computing, Zhejiang Laboratory, Hangzhou, People’s Republic of China

    Tianwei Zhang

  9. National Astronomical Observatory of Japan, Tokyo, Japan

    Yu Cheng

  10. School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai, People’s Republic of China

    Yihuan Di

  11. Department of Physics, Anhui Normal University, Wuhu, People’s Republic of China

    Ningyu Tang

  12. National Astronomical Observatories, Chinese Academy of Sciences, Beijing, People’s Republic of China

    Aiyuan Yang

Authors
  1. Xunchuan Liu
    View author publications

    Search author on:PubMed Google Scholar

  2. Tie Liu
    View author publications

    Search author on:PubMed Google Scholar

  3. Pak-Shing Li
    View author publications

    Search author on:PubMed Google Scholar

  4. Xiaofeng Mai
    View author publications

    Search author on:PubMed Google Scholar

  5. Christian Henkel
    View author publications

    Search author on:PubMed Google Scholar

  6. Paul F. Goldsmith
    View author publications

    Search author on:PubMed Google Scholar

  7. Sheng-Li Qin
    View author publications

    Search author on:PubMed Google Scholar

  8. Yan Gong
    View author publications

    Search author on:PubMed Google Scholar

  9. Xing Lu
    View author publications

    Search author on:PubMed Google Scholar

  10. Fengwei Xu
    View author publications

    Search author on:PubMed Google Scholar

  11. Qiuyi Luo
    View author publications

    Search author on:PubMed Google Scholar

  12. Hong-Li Liu
    View author publications

    Search author on:PubMed Google Scholar

  13. Tianwei Zhang
    View author publications

    Search author on:PubMed Google Scholar

  14. Yu Cheng
    View author publications

    Search author on:PubMed Google Scholar

  15. Yihuan Di
    View author publications

    Search author on:PubMed Google Scholar

  16. Yuefang Wu
    View author publications

    Search author on:PubMed Google Scholar

  17. Qilao Gu
    View author publications

    Search author on:PubMed Google Scholar

  18. Ningyu Tang
    View author publications

    Search author on:PubMed Google Scholar

  19. Aiyuan Yang
    View author publications

    Search author on:PubMed Google Scholar

  20. Zhiqiang Shen
    View author publications

    Search author on:PubMed Google Scholar

Contributions

X. Liu was the principal investigator of the FAST project. He led the data reduction, filament-extracting code development and scientific analysis, and wrote the manuscript. T.L. contributed to the FAST proposal, the interpretation of results and the writing of the manuscript. P.S.-L. conducted the MHD simulation. All authors participated in the discussion of results and revision of the manuscript.

Corresponding authors

Correspondence to Xunchuan Liu, Tie Liu or Zhiqiang Shen.

Ethics declarations

Competing interests

The authors declare no competing interests.

Peer review

Peer review information

Nature Astronomy thanks the anonymous reviewers for their contribution to the peer review of this work.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Location and scanning pattern of the G165 VHVC.

Upper panel: Color map composed of the H I emission from three different velocity ranges from the HI4PI survey40. Prominent HVC complexes (AC complex, Cohen Stream, Giovanelli Stream, and MS) and external galaxies (LMC, SMC, M33, and M31) are labeled. Lower left panel: Zoom-in image of the G165 VHVC from the HI4PI survey. The orange rectangles (1∘ × 1∘ each) mark the ten on-the-fly observation fields of FAST. The white and red circles represent the beam size of HI4PI and FAST, respectively. Lower right panel: Effective integration time of the FAST observations.

Source data

Extended Data Fig. 2 Comparison between the FAST observations and archival HI4PI data.

Upper panels: The integrated intensity maps of the HVC and foreground (fg) components of G165 by FAST and HI4PI. See Fig. 1 and Extended Data Fig. 1 for the integrated intensity maps of the VHVC component. Center panels: Channel maps of G165 VHVC by FAST with velocity resolution smoothed to 2 km/s (about one fiftieth of the velocity span). Lower panels: Channel maps of the foreground of G165 by FAST with velocity resolution smoothed to 0.3 km/s (about one fiftieth of the velocity span).

Source data

Extended Data Fig. 3 Comparison between the filaments of the G165 VHVC extracted using different algorithms.

The left panel shows the result from FilFinder, and the right panel shows the result from ClimbRidge. The background in both panels is the integrated intensity map of the G165 VHVC, processed with the Sharpen operation from the scikit-image Python package to enhance the filamentary structures (see Sect. Methods).

Source data

Extended Data Fig. 4 Filaments extracted using ClimbRidge in the ppv cube of the G165 VHVC, viewed from different projection angles.

The projections are along the three axes: RA, DEC, and VLSR. See Fig. 2 for off-axis projections. Three representative filaments, B1, B2, and B3, are shown in different colors (see Fig. 2). In the bottom-right panel, the red dotted and dashed lines separate the three velocity layers (L1, L2, and L3) of the G165 VHVC. The red dashed line has a slope of 0.3 km/s per arcmin.

Source data

Extended Data Fig. 5 Minimal spanning tree (MST) analysis of the H I segments of the G165 VHVC.

Left and Middle panels: The MSTs of the density ridge points extracted by ClimbRidge in the 3D ppv cube (left) and on the 2D integrated intensity map of the G165 VHVC. Different colors represent different segments connecting every two neighboring nodes of the MSTs. Right panel: The PDFs of the segment lengths.

Source data

Extended Data Fig. 6 Three example H I filaments of the G165 VHVC.

(a) Zoom-in images of the three filaments shown in Extended Data Fig. 4. (b) Slice in the l-VLSR plane, where l is the length along the ridge. (c) Slice in the l-r plane, where r is the radial offset from the ridge. (d) Radial intensity profiles (gray lines) and the mean profile (black line). The red dashed line shows the fitting result of the black line using a Gaussian function plus a first-order polynomial.

Source data

Extended Data Fig. 7 The integrated intensity maps of the MHD simulation.

In each panel, the integrated intensity has been normalized by its mean value. The filaments of the best-fit model (Ms = 3 and MA = 1) were extracted in the ppv cube (Supplementary Fig. 1) using ClimbRidge and shown as red lines.

Source data

Extended Data Fig. 8 The column density PDFs of the decomposed Gaussian components of the simulated ppv cubes.

The blue lines are the simulated PDFs. The red dotted line represents the fourth-order polynomial fitting of the observational result for the G165 VHVC (Fig. 3).

Source data

Extended Data Table 1 Fitted parameters of the typical filaments
Full size table

Supplementary information

Supplementary Information

Supplementary Figs. 1–5.

Source data

Source Data Fig. 1

FITS file of the zeroth-moment (mom0) map of G165 VHVC. Uploaded to https://zenodo.org/records/15347725.

Source Data Fig. 2

The raw output of the Gaussian decomposition of the observed H i data using scousepy.

Source Data Fig. 3

The raw output of the Gaussian decomposition of the best-fit simulation (scaleless version) using scousepy.

Source Data Extended Data Fig. 1

Set-up of FAST observation.

Source Data Extended Data Fig. 2

FITS cube of G165 VHVC by FAST (smoothed to a velocity resolution of 1 km s−1). Uploaded to https://zenodo.org/records/15347725.

Source Data Extended Data Fig. 3

Filaments of the observed H i mom0 map extracted using FilFinder.

Source Data Extended Data Fig. 4

Filaments of the observed H i ppv cube extracted using ClimbRidge.

Source Data Extended Data Fig. 5

Filaments of the observed H i mom0 map extracted using ClimbRidge.

Source Data Extended Data Fig. 6

Locations of the three branches.

Source Data Extended Data Fig. 7

Mom0 map of the best-fit simulation (scaleless version). Uploaded to https://zenodo.org/records/15347725.

Source Data Extended Data Fig. 8

Column density distribution of the best-fit simulation.

Source Data Extended Data Table 1

Readable table.

Rights and permissions

Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Liu, X., Liu, T., Li, PS. et al. A network of velocity-coherent filaments formed by supersonic turbulence in a very-high-velocity H i cloud. Nat Astron 9, 1366–1374 (2025). https://doi.org/10.1038/s41550-025-02605-8

Download citation

  • Received:

  • Accepted:

  • Published:

  • Version of record:

  • Issue date:

  • DOI: https://doi.org/10.1038/s41550-025-02605-8

Search

Advanced search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing