The density structure of the interstellar medium determines where stars form and release energy, momentum and heavy elements, driving galaxy evolution^1,2,3,4. 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 instabilities^6,7, 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 measure^10,11,12 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 instabilities^13,14. 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.

星际介质的密度结构决定了恒星在何处形成并释放能量，动量和重元素，从而驱动银河系^1,2,3,4。密度变化是通过气体运动来播种和放大的，但是这种运动的确切性质在整个空间尺度和银河环境中都是未知的⁵。尽管稠密的恒星形成气体可能是由不稳定性^6,7，会聚流⁸和湍流⁹的组合产生的，但要确定精确的起源是有挑战性的，因为它需要在空间尺度上量化多个数量级的气体运动。在这里我们测量^10,11,12分子气体在银河系和附近星系NGC 4321中的运动，集合了横跨空间动态范围10 ^-1 –10 ³  pc的观测结果。我们检测到所有空间尺度和银河环境中普遍存在的速度波动。对这些波动的统计分析表明了如何形成恒星气体。我们发现振荡气流的波长范围为0.3–400 pc。这些流动耦合到可能通过重力不稳定性^13,14形成的规则间隔的密度增强。我们还确定了随机和无标度的速度和密度波动，与湍流中产生的结构一致⁹。我们的研究结果表明，不能孤立地考虑星际介质的结构。取而代之的是，它的形成和演化是由嵌套的，相互依存的物质流控制的，这些物质流在空间尺度上覆盖了多个数量级。