Power-law tails in probability density functions of molecular cloud column density

TL;DR

This study demonstrates that molecular cloud column density PDFs are predominantly lognormal when carefully selecting cold, molecular regions and accounting for noise, challenging the common attribution of power-law tails solely to gravity.

Contribution

The paper shows that apparent power-law tails in molecular cloud PDFs are often due to noise, contamination, or inclusion of warm material, emphasizing the importance of proper data selection and analysis.

Findings

Cold, molecular regions exhibit lognormal PDFs.

Noise and contamination can create or exaggerate tails.

Proper data handling reduces apparent power-law tails.

Abstract

Power-law tails are often seen in probability density functions (PDFs) of molecular cloud column densities, and have been attributed to the effect of gravity. We show that extinction PDFs of a sample of five molecular clouds obtained at a few tenths of a parsec resolution, probing extinctions up to A$_{{\mathrm{V}}}$ $\sim$ 10 magnitudes, are very well described by lognormal functions provided that the field selection is tightly constrained to the cold, molecular zone and that noise and foreground contamination are appropriately accounted for. In general, field selections that incorporate warm, diffuse material in addition to the cold, molecular material will display apparent core+tail PDFs. The apparent tail, however, is best understood as the high extinction part of a lognormal PDF arising from the cold, molecular part of the cloud. We also describe the effects of noise and foreground/background contamination on the PDF structure, and show that these can, if not appropriately accounted for, induce spurious tails or amplify any that are truly present.