Gravity or turbulence? II. Evolving column density PDFs in molecular clouds

TL;DR

This paper demonstrates through simulations that the shape of the column density PDF in molecular clouds evolves from lognormal to power-law as gravitational contraction progresses, linking cloud dynamics to observed PDF features.

Contribution

It provides a unified explanation for the evolution of column density PDFs in molecular clouds based on their gravitational contraction stages, supported by thermally bi-stable simulations.

Findings

Npdf evolves from narrow lognormal to wider lognormal and then to power-law during cloud evolution.

The power-law slope varies depending on the observational projection.

Cloud dynamics are primarily driven by gravitational contraction, influencing Npdf shape.

Abstract

It has been recently shown that molecular clouds do not exhibit a unique shape for the column density probability distribution function (Npdf). Instead, clouds without star formation seem to possess a lognormal distribution, while clouds with active star formation develope a power-law tail at high column densities. The lognormal behavior of the Npdf has been interpreted in terms of turbulent motions dominating the dynamics of the clouds, while the power-law behavior 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 Npdf, and later, in a power-law Npdf. We thus suggest that the observed Npdf 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.