2017MNRAS.466.4633A

Recently reported variations in the typical physical properties of Galactic and extra-Galactic molecular clouds (MCs), and, in their star-forming ability, have been attributed to local variations in the magnitude of interstellar pressure. Inferences from these surveys have called into question two long-standing beliefs that: (1) MCs are virialized and (2) they obey the Larson's third law. Here we invoked the framework of cloud formation via collision between warm gas-flows to examine if these latest observational inferences can be reconciled. To this end, we traced the temporal evolution of the gas surface density, the fraction of dense gas, the distribution of gas column density (N-PDF) and the virial nature of the assembled clouds. We conclude that these physical properties exhibit temporal variation and their respective peak magnitude also increases in proportion with the magnitude of external pressure, P_ext. The velocity dispersion in assembled clouds appears to follow the power law, σ_gas∝P_ext^0.23. The power-law tail of the N-PDFs at higher densities becomes shallower with increasing magnitude of external pressure for P_ext/k_B <= 10⁷ K cm^–3; at higher magnitudes such as those typically found in the Galactic Central Molecular Zone (P_ext/k_B > 10⁷ K cm^–3), the power-law shows significant steepening. While our results are broadly consistent with inferences from various recent observational surveys, it appears that MCs do not exhibit a unique set of properties, but rather a wide variety that can be reconciled with a range of magnitudes of pressure between 10⁴ and 10⁸ K cm^–3.