Gravity or turbulence? – II. Evolving column density probability distribution functions in molecular clouds

Abstract

It has been recently shown that molecular clouds do not exhibit a unique shape for the column density probability distribution function ( N ‐PDF). Instead, clouds without star formation seem to possess a lognormal distribution, while clouds with active star formation develop a power‐law tail at high column densities. The lognormal behaviour of the N ‐PDF has been interpreted in terms of turbulent motions dominating the dynamics of the clouds, while the power‐law behaviour occurs when the cloud is dominated by gravity. In the present contribution, we use thermally bi‐stable numerical simulations of cloud formation and evolution to show that, indeed, these two regimes can be understood in terms of the formation and evolution of molecular clouds: a very narrow lognormal regime appears when the cloud is being assembled. However, as the global gravitational contraction occurs, the initial density fluctuations are enhanced, resulting, first, in a wider lognormal N ‐PDF, and later, in a power‐law N ‐PDF. We thus suggest that the observed N ‐PDF of molecular clouds are a manifestation of their global gravitationally contracting state. We also show that, contrary to recent suggestions, the exact value of the power‐law slope is not unique, as it depends on the projection in which the cloud is being observed.