Relationship between the column density distribution and evolutionary class of molecular clouds as viewed by ATLASGAL

TL;DR

This study investigates how the column density distribution of molecular clouds correlates with their evolutionary stages, revealing distinct N-PDF shapes for different classes and linking dense gas content to cloud mass.

Contribution

It provides the first analysis connecting N-PDF shapes with evolutionary classes of molecular clouds using ATLASGAL data.

Findings

Starless clumps have narrow, log-normal N-PDFs.

Star-forming clouds and HII regions show power-law N-PDFs.

Dense gas mass fraction correlates with total cloud mass.

Abstract

We present the first study of the relationship between the column density distribution of molecular clouds within nearby Galactic spiral arms and their evolutionary status as measured from their stellar content. We analyze a sample of 195 molecular clouds located at distances below 5.5 kpc, identified from the ATLASGAL 870 micron data. We define three evolutionary classes within this sample: starless clumps, star-forming clouds with associated young stellar objects, and clouds associated with HII regions. We find that the N(H2) probability density functions (N-PDFs) of these three classes of objects are clearly different: the N-PDFs of starless clumps are narrowest and close to log-normal in shape, while star-forming clouds and HII regions exhibit a power-law shape over a wide range of column densities and log-normal-like components only at low column densities. We use the N-PDFs to estimate the evolutionary time-scales of the three classes of objects based on a simple analytic model from literature. Finally, we show that the integral of the N-PDFs, the dense gas mass fraction, depends on the total mass of the regions as measured by ATLASGAL: more massive clouds contain greater relative amounts of dense gas across all evolutionary classes.