Evolving Molecular Cloud Structure and the Column Density Probability Distribution Function

TL;DR

This study investigates how the structure of molecular clouds, as characterized by the column density PDF, evolves over time and relates to star formation, explaining observational differences in cloud properties.

Contribution

The paper introduces a model showing the evolution of the column density PDF in molecular clouds and links its features to cloud age and star formation activity.

Findings

Peak of the PDF shifts below detection threshold in evolved clouds

Lognormal distribution becomes indistinguishable at late times

Power-law tail properties stabilize, correlating with star formation thresholds

Abstract

The structure of molecular clouds can be characterized with the probability distribution function (PDF) of the mass surface density. In particular, the properties of the distribution can reveal the nature of the turbulence and star formation present inside the molecular cloud. In this paper, we explore how these structural characteristics evolve with time and also how they relate to various cloud properties as measured from a sample of synthetic column density maps of molecular clouds. We find that, as a cloud evolves, the peak of its column density PDF will shift to surface densities below the observational threshold for detection, resulting in an underlying lognormal distribution which has been effectively lost at late times. Our results explain why certain observations of actively star-forming, dynamically older clouds, such as the Orion molecular cloud, do not appear to have any evidence of a lognormal distribution in their column density PDFs. We also study the evolution of the slope and deviation point of the power-law tails for our sample of simulated clouds and show that both properties trend towards constant values, thus linking the column density structure of the molecular cloud to the surface density threshold for star formation.