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\nshape for the column density probability distribution function (Npdf). Instead,\nclouds without star formation seem to possess a lognormal distribution, while\nclouds with active star formation develope a power-law tail at high column\ndensities. The lognormal behavior of the Npdf has been interpreted in terms of\nturbulent motions dominating the dynamics of the clouds, while the power-law\nbehavior occurs when the cloud is dominated by gravity. In the present\ncontribution we use thermally bi-stable numerical simulations of cloud\nformation and evolution to show that, indeed, these two regimes can be\nunderstood in terms of the formation and evolution of molecular clouds: a very\nnarrow lognormal regime appears when the cloud is being assembled. However, as\nthe global gravitational contraction occurs, the initial density fluctuations\nare enhanced, resulting, first, in a wider lognormal Npdf, and later, in a\npower-law Npdf. We thus suggest that the observed Npdf of molecular clouds are\na manifestation of their global gravitationally contracting state. We also show\nthat, contrary to recent suggestions, the exact value of the power-law slope is\nnot unique, as it depends on the projection in which the cloud is being\nobserved.\n