Ubiquitous velocity fluctuations throughout the molecular interstellar medium
Statistical analysis of velocity fluctuations in the interstellar medium (ISM) of the Milky Way and NGC 4321 show that the motion of molecular gas over scales ranging from 0.1 to 1,000 pc is similar, and consistent with that generated by a combination of gravity and turbulence. ISM structure at one scale is therefore linked to structure at other scales. The density structure of the interstellar medium determines where stars form and release energy, momentum and heavy elements, driving galaxy evolution(1-4). Density variations are seeded and amplified by gas motion, but the exact nature of this motion is unknown across spatial scales and galactic environments(5). Although dense star-forming gas probably emerges from a combination of instabilities(6,7), convergent flows(8)and turbulence(9), establishing the precise origin is challenging because it requires gas motion to be quantified over many orders of magnitude in spatial scale. Here we measure(10-12)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)-10(3) 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 instabilities(13,14). We also identify stochastic and scale-free velocity and density fluctuations, consistent with the structure generated in turbulent flows(9). 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. ; German Research Foundation (DFG) KR4801/1-1 KR4801/2-1 European Research Council (ERC) 714907 European Union's Horizon 2020 research and innovation program 639459 National Science Foundation (NSF) 1615105 1615109 1653300 NASA under ADAP NNX16AF48G NNX17AF39G Natural Sciences and Engineering Research Council of Canada RGPIN-2017-03987 National Science Foundation (NSF) 1816715 AST-9800334 AST-0098562 AST-0100793 AST-0228993 AST-0507657 German Research Foundation (DFG) SFB 881 Heidelberg Cluster of Excellence STRUCTURES of Germany's Excellence Strategy EXC-2181/1-390900948 ERC under the European Union's Horizon 2020 research and innovation programme 694343 European Union's Horizon 2020 research and innovation programme 726384 Programme National 'Physique et Chimie du Milieu Interstellaire' (PCMI) of CNRS/INSU INC/INP French Atomic Energy Commission Centre National D'etudes Spatiales Australian Government Australian Research Council UNSW, Sydney Monash Universities Commonwealth Scientific & Industrial Research Organisation (CSIRO)