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Ubiquitous velocity fluctuations throughout the molecular interstellar medium

Abstract

The density structure of the interstellar medium determines where stars form and release energy, momentum and heavy elements, driving galaxy evolution1,2,3,4. Density variations are seeded and amplified by gas motion, but the exact nature of this motion is unknown across spatial scales and galactic environments5. Although dense star-forming gas probably emerges from a combination of instabilities6,7, convergent flows8 and turbulence9, establishing the precise origin is challenging because it requires gas motion to be quantified over many orders of magnitude in spatial scale. Here we measure10,11,12 the motion of molecular gas in the Milky Way and in nearby galaxy NGC 4321, assembling observations that span a spatial dynamic range 10−1–103 pc. We detect ubiquitous velocity fluctuations across all spatial scales and galactic environments. Statistical analysis of these fluctuations indicates how star-forming gas is assembled. We discover oscillatory gas flows with wavelengths ranging from 0.3–400 pc. These flows are coupled to regularly spaced density enhancements that probably form via gravitational instabilities13,14. We also identify stochastic and scale-free velocity and density fluctuations, consistent with the structure generated in turbulent flows9. Our results demonstrate that the structure of the interstellar medium cannot be considered in isolation. Instead, its formation and evolution are controlled by nested, interdependent flows of matter covering many orders of magnitude in spatial scale.

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Fig. 1: Ubiquitous velocity fluctuations throughout the molecular ISM.
Fig. 2: Correlated density and velocity fluctuations.

Data availability

The 13CO (1−0) data of the Galactic Disk are from the Boston University-FCRAO GRS. The GRS data are publicly available at https://www.bu.edu/galacticring/new_data.html. The Mopra data are publicly available at http://newt.phys.unsw.edu.au/mopracmz/data.html. The ALMA HNCO 4(0,4) − 3(0,3) data of G0.253+0.016 are from project 2011.0.00217.S (principal investigator: J. Rathborne) and the raw data are publicly available through the ALMA archive (https://almascience.eso.org/alma-data/archive). All other data that support the findings of this study are available from the corresponding author on reasonable request. Source data are provided with this paper.

Code availability

SCOUSEPY and ACORNS, as well as the codes used for our statistical analyses, are freely available at https://github.com/jdhenshaw. GAUSSPY+ is available at https://github.com/mriener/gausspyplus. Assistance with this software can be provided by the corresponding author.

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Acknowledgements

We thank T. Müller from the Max Planck Institut für Astronomie for assisting with the data visualization and production of the Supplementary Videos. We thank J. Rathborne for making the data on G0.253+0.016 available, and P. Caselli, B. Elmegreen and J. Soler for discussions. J.M.D.K. and M.C. acknowledge funding from the German Research Foundation (DFG) in the form of an Emmy Noether Research Group grant (number KR4801/1-1) and DFG Sachbeihilfe grant (number KR4801/2-1). J.M.D.K. acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme via the ERC Starting Grant MUSTANG (grant agreement number 714907). M.R. and J.K. acknowledge funding from the European Union’s Horizon 2020 research and innovation program under grant agreement number 639459 (PROMISE). The work of A.K.L. and J.S. is partially supported by the National Science Foundation (NSF) under grant numbers 1615105, 1615109 and 1653300, and by NASA under ADAP grant numbers NNX16AF48G and NNX17AF39G. E.R. acknowledges the support of the Natural Sciences and Engineering Research Council of Canada (NSERC) under funding reference number RGPIN-2017-03987. C.B. acknowledges support from the NSF under grant number 1816715. R.S.K. and S.C.O.G. acknowledge funding from the Deutsche Forschungsgemeinschaft (DFG) via the Collaborative Research Center (SFB 881 The Milky Way System; subprojects A1, B1, and B2) and from the Heidelberg Cluster of Excellence STRUCTURES in the framework of Germany’s Excellence Strategy (grant number EXC-2181/1-390900948). E.S. acknowledges funding from the ERC under the European Union’s Horizon 2020 research and innovation programme (grant agreement number 694343). F.B. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme (grant agreement number 726384). J.P. acknowledges funding from the Programme National ‘Physique et Chimie du Milieu Interstellaire’ (PCMI) of CNRS/INSU with INC/INP, co-funded by CEA and CNES. The GRS is a joint project of Boston University and Five College Radio Astronomy Observatory, funded by the NSF under grant numbers AST-9800334, AST-0098562, AST-0100793, AST-0228993 and AST-0507657. The N2H+ (1−0) data of the CMZ was obtained using the Mopra radio telescope, a part of the Australia Telescope National Facility which is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO. The University of New South Wales (UNSW) digital filter bank (the UNSW-MOPS) used for the observations with Mopra was provided with support from the Australian Research Council (ARC), UNSW, Sydney and Monash Universities, as well as CSIRO.

Author information

Affiliations

Authors

Contributions

J.D.H., J.M.D.K. and S.N.L. were responsible for the experiment design. J.D.H. led the project, carried out the experiment, developed the analysis methods, interpreted the results and wrote the text, to which J.M.D.K, S.N.L. and M.R. contributed. E.R. created the mock datasets and designed the observational estimator 2D model test. J.D.H. and M.R. were responsible for data visualization. Supplementary Videos 1–5 were coproduced by J.D.H. Supplementary Videos 610 were produced by M.R. J.D.H. performed the spectral decomposition for all regions except for the GRS data for which M.R. was responsible. All authors contributed to aspects of the data reduction and analysis, the interpretation of the results, and the writing of the manuscript.

Corresponding author

Correspondence to Jonathan D. Henshaw.

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Competing interests

The authors declare no competing interests.

Additional information

Peer review information Nature Astronomy thanks Mark Heyer and Alex Lazarian for their contribution to the peer review of this work.

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

Extended data

Extended Data Fig. 1 Summary of the observations.

Here we highlight the observations and region selection for the data presented in Figure 1. The scales probed by our Galactic disc selection (seen in square brackets) are relevant for a distance of 3 kpc. Out of each of these environments we select sub-regions for the statistical analysis presented in Fig. 2 (see Statistical analysis of the observational data).

Extended Data Fig. 2 Maps of the regions selected for statistical analysis.

The upper panels display our galactic disc environments. From left to right we show part of the main southern spiral arm in NGC 4321 (a), a GMC in the Galactic disc (b), and an individual filament located within that same GMC (c). The bottom panels display our selected regions in the CMZ: The series of molecular clouds investigated by ref. 13 (d) and an individual GMC located within the CMZ gas stream (e). The cyan points in panel ‘a’ refer to the locations of star forming complexes identified in the mid-infrared14. In the upper left of each panel we indicate the tracer used to create each image. Scale bars are included in the bottom right corner of each image. These regions correspond to the areas over which we perform our statistical analysis (see Statistical analysis of the observational data and Fig. 2).

Extended Data Fig. 3 Distribution of our density proxy (top) and velocity centroids (bottom) along the crest of the structures displaying periodicity.

From left to right we show distance along the crest of each structure versus mean density (top) and velocity (bottom), for our selected regions in NGC 4321 (a, b), the CMZ (c, d), and IRDC G035.39-00.33 (e, f), respectively (see Extended Data Fig. 2). The coloured shaded region in each panel represents the standard deviation of the data measured orthogonal to the crest. Source data

Extended Data Fig. 4 A comparison between our density proxy and the line-of-sight velocity differential.

Here we show the profile of our density proxy (coloured lines) with the normalised velocity differential (black line) along the crests of our our selected regions in NGC 4321 (a), the CMZ (b), and IRDC G035.39-00.33 (c), respectively. Note that in panel ‘b’ we show emission of N2H+ (1-0) rather than the column density distribution displayed in Extended Data Fig. 3c (see the discussion in the Supplementary Information). The black dotted line highlights where the derivative of the velocity is 0.0 kms−1 pc−1.

Supplementary information

Supplementary Information

Supplementary Figs. 1– 6, models and discussion.

Supplementary Video 1

Ubiquitous velocity fluctuations throughout the molecular interstellar medium. Here we show a movie highlighting the p-p-v structure of NGC4321. The data points are equivalent to those presented in Fig. 1a.

Supplementary Video 2

Equivalent to Supplementary Video 1, but for the Galactic Disk.

Supplementary Video 3

Equivalent to Supplementary Video 1, but for the G035.39-00.33.

Supplementary Video 4

Equivalent to Supplementary Video 1, but for the CMZ.

Supplementary Video 5

Equivalent to Supplementary Video 1, but for G0.253+0.016.

Supplementary Video 6

A movie highlighting the velocity fluctuations throughout NGC4321. Here we show the position-velocity location of data extracted using our spectral decomposition method for individual slices through the other position axis.

Supplementary Video 7

Equivalent to Supplementary Video 6, but for the Galactic disc. Here we show the full decomposition of the GRS data set and not just the region displayed in Fig. 1b and in Supplementary Video 2. This demonstrates that the velocity fluctuations are detected throughout the entire area covered by the GRS survey.

Supplementary Video 8

Equivalent to Supplementary Video 6, but for G035.39-00.33.

Supplementary Video 9

Equivalent to Supplementary Video 6, but for the CMZ.

Supplementary Video 10

Equivalent to Supplementary Video 6, but for G0.253+0.016.

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Source Data Fig. 2

Source data for Fig. 2.

Source Data Extended Data Fig. 3

Source data for Extended Data Fig. 3.

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Henshaw, J.D., Kruijssen, J.M.D., Longmore, S.N. et al. Ubiquitous velocity fluctuations throughout the molecular interstellar medium. Nat Astron 4, 1064–1071 (2020). https://doi.org/10.1038/s41550-020-1126-z

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Further reading

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