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arXiv · 2607.24977

The $\sigma_s^2-\mathcal{M}$ relation in the multi-phase ISM: Exploring the density PDF with the Cloud Factory simulations

Mairi Nonhebel·Rowan J. Smith·Ralf S. Klessen
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Abstract

The density probability distribution (PDF) of molecular clouds is a crucial component of analytical theories of star formation. In idealised simulations of isothermal turbulence, the width of the density PDF, $\sigma_s^2$, is dependent on the sonic Mach number of the medium, $\mathcal{M}$. The $\sigma_s^2-\mathcal{M}$ relation is widely used to connect cloud-scale turbulence to the density PDF, and further to star formation activity, yet its validity within individual phases of the multi-phase interstellar medium (ISM) remains untested. In this study, we evaluate whether the $\sigma_s^2-\mathcal{M}$ relation is applicable to individual phases of the ISM. We study the density PDFs of molecular cloud complexes in the Cloud Factory simulations; a suite of detailed zoom-in simulations that self-consistently generate a turbulent, multi-phase ISM. We test whether the $\sigma_s^2-\mathcal{M}$ relation holds in the hot ionised medium (HIM), warm ionised medium (WIM), warm neutral medium (WNM), cold neutral medium (CNM), the molecular phase, and the highly-shielded molecular phase traced by CO. We find the applicability of the classical $\sigma_s^2-\mathcal{M}$ relation to vary between phases and depend strongly on how $\sigma_s^2$ and $\mathcal{M}$ are measured. The relation fails to capture the widths of the WNM and CNM density distributions, with possible contributing factors including non-isothermality and large-scale coherent motions. In contrast, we find the $\sigma_s^2-\mathcal{M}$ relation to tentatively hold for the log-normal portion of the H$_2$ distribution. The width of the CO density PDF is systematically overpredicted by the classical relation, resulting from the selective nature of CO as a molecular gas tracer.

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Mairi Nonhebel, Rowan J. Smith, Ralf S. Klessen. 2026-07-27. The $\sigma_s^2-\mathcal{M}$ relation in the multi-phase ISM: Exploring the density PDF with the Cloud Factory simulations. https://arxiv.org/abs/2607.24977

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