Molecular clouds have power-law probability distribution functions

¹ University of Milan, Department of Physics, via Celoria 16, 20133 Milan, Italy
e-mail: marco.lombardi@unimi.it
² University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
³ Harvard-Smithsonian Center for Astrophysics, Mail Stop 72, 60 Garden Street, Cambridge, MA 02138, USA

Received: 12 January 2015
Accepted: 9 February 2015

Abstract

In this Letter we investigate the shape of the probability distribution of column densities (PDF) in molecular clouds. Through the use of low-noise, extinction-calibrated Herschel/Planck emission data for eight molecular clouds, we demonstrate that, contrary to common belief, the PDFs of molecular clouds are not described well by log-normal functions, but are instead power laws with exponents close to two and with breaks between A_K ≃ 0.1 and 0.2 mag, so close to the CO self-shielding limit and not far from the transition between molecular and atomic gas. Additionally, we argue that the intrinsic functional form of the PDF cannot be securely determined below A_K ≃ 0.1 mag, limiting our ability to investigate more complex models for the shape of the cloud PDF.