Ubiquitous velocity fluctuations throughout the molecular interstellar medium

The density structure of the interstellar medium (ISM) determines where stars form and release energy, momentum, and heavy elements, driving galaxy evolution. Density variations are seeded and amplified by gas motion, but the exact nature of this motion is unknown across spatial scale and galactic environment. Although dense star-forming gas probably emerges from a combination of instabilities, convergent flows, and turbulence, establishing the precise origin is challenging because it requires quantifying gas motion over many orders of magnitude in spatial scale. In this publication, we measure the motion of molecular gas in the Milky Way and in nearby galaxy NGC 4321, thus assembling observations that span an unprecedented spatial dynamic range (0.1pc to >1000pc). We detect ubiquitous velocity fluctuations across all spatial scales and galactic environments. These velocity fluctuations, reminiscent in appearance of waves on the surface of the ocean (see the below figure), represent localised gas flows. Statistical analysis of the fluctuations indicates how star-forming gas is assembled. We discover oscillatory gas flows with wavelengths ranging from 0.3pc to 400pc. These flows are coupled to regularly-spaced density enhancements that probably form via gravitational instabilities. We also identify stochastic and scale-free velocity and density fluctuations, consistent with the structure generated in turbulent flows. Our results demonstrate that ISM structure cannot be considered in isolation. Instead, its formation and evolution is controlled by nested, interdependent flows of matter covering many orders of magnitude in spatial scale.

J. D. Henshaw et al., 2020, Nature Astronomy

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