The sigma s 2-M relation in the multi-phase ISM: Exploring the density PDF with the Cloud Factory simulations

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 squared, is dependent on the sonic Mach number of the medium, M. The sigma s 2-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-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-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-M relation to vary between phases and depend strongly on how sigma s squared and 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-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.