A measurement of the turbulence-driven density distribution in a non-star-forming molecular cloud

Abstract: Molecular clouds are supersonically turbulent. This turbulence governs the initial mass function and the star formation rate. In order to understand the details of star formation, it is therefore essential to understand the properties of turbulence, in particular the probability distribution of density in turbulent clouds. We present formaldehyde volume density measurements of a non-star-forming cloud along the line of sight towards W49A. We use these measurements in conjunction with total mass estimates from 13CO to infer the shape of the density probability distribution function. This method is complementary to measurements of turbulence via the column density distribution and should be applicable to any molecular cloud with detected CO. We show that turbulence in this cloud is probably compressively driven, with a compressive-to-total Mach number ratio $b = \mathcal{M}_C/\mathcal{M}>0.4$. We measure the standard deviation of the density distribution, constraining it to the range $1.5 < \sigma_s < 1.9$ assuming that the density is lognormally distributed. This measurement represents an essential input into star formation laws. The method of averaging over different excitation conditions to produce a model of emission from a turbulent cloud is generally applicable to optically thin line observations.