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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- September 2011
- DOI:
- 10.1111/j.1365-2966.2011.19141.x
- arXiv:
- arXiv:1105.5411
- Bibcode:
- 2011MNRAS.416.1436B
- Keywords:
-
- turbulence;
- stars: formation;
- ISM: clouds;
- ISM: general;
- ISM: kinematics and dynamics;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 9 pages, 4 figures. MNRAS, accepted