The Structure and Evolution of Unbound Star-forming Molecular Clouds

Abstract

Recent generations of stars form principally, and possibly exclusively, in giant molecular clouds -- large conglomerations of gas and dust primarily composed of molecular hydrogen and concentrated in the arms of spiral galaxies. These clouds are assumed to be gravitationally bound; however, recent observations suggest the presence of a substantial population of unbound clouds in the Milky Way. Using synthetic observations from high-resolution simulations of bound and unbound molecular clouds, we explore whether clouds in this mixed population could match observations of local molecular clouds. We find from the clouds in our sample that a state of virial equilibrium is not required to form stars and match the dynamics and structure of observed clouds, as described by the Larson scaling relations and the probability distribution function (PDF) of the mass surface density. As these clouds evolve, the underlying lognormal shape of the column density PDFs is effectively concealed as the peaks of their distributions shift to surface densities below observational detection thresholds, supporting recent observations which also find little to no evidence for a lognormal distribution in column density PDFs of nearby clouds. We explore these results further in an extragalactic context by simulating molecular clouds formed in a galactic disc, in order to demonstrate the role their environment, particularly the galactic shear, plays on their structure and evolution and on the star formation within them. We find that a substantial population of unbound molecular clouds forms naturally in a galactic disc environment and demonstrate that their presence not only matches galactic and extragalactic observations but also impacts several long-standing issues in star formation.