Systematic Investigation of Dust and Gaseous CO in 12 Nearby Molecular Clouds

TL;DR

This study systematically analyzes dust and CO in 12 nearby molecular clouds, revealing consistent cloud properties, the limitations of CO as a tracer, and emphasizing the importance of standardized boundary definitions.

Contribution

It provides the first uniform analysis of dust and molecular gas in local clouds, establishing key relationships and evaluating CO's effectiveness as a tracer.

Findings

Average dust opacity is 0.8 cm^2/g with variations of factor 2.

Cloud PDFs follow steep power-law distributions.

Cloud mass scales with the square of radius, indicating constant surface density.

Abstract

We report the first uniform and systematic study of dust and molecular gas in nearby molecular clouds. We use surveys of dust extinction and emission to determine the opacity and map the distribution of the dust within a dozen local clouds in order to derive a uniform set of basic cloud properties. We find: 1) the average dust opacity $\langle\kappa_{d,353}\rangle = 0.8\ {\rm cm^{2}\, g^{-1}}$ with variations of a factor of $\sim$ 2 between clouds, 2) cloud PDFs are exquisitely described by steeply falling power-laws with a narrow range of slope, and 3) a tight $M_{\rm GMC} \sim R_{\rm GMC}^2$ scaling relation for the cloud sample, indicative of a cloud population with an exactingly constant average surface density above a common fixed boundary. We compare these results to uniformly analyzed CO surveys. We measure the CO mass conversion factors and assess the efficacy of CO for tracing the physical properties of molecular clouds. We find $\langle \alpha_{\rm CO}\rangle = 4.31 \pm 0.67$ M$_\odot$ (K km s$^{-1}$ pc$^2$)$^{-1}$ (corresponding to $X_{\rm CO}$ = 1.97 $\times$ 10$^{20}$ cm$^{-2}$(K km s$^{-1}$)$^{-1}$). We demonstrate that CO observations are a poor tracer of column density and structure on sub-cloud spatial scales. On cloud scales, CO observations can provide measurements consistent with those of the dust, provided data are analyzed in a similar, self-consistent fashion. Measurements of average GMC surface density are sensitive to choice of cloud boundary. Care must be exercised to adopt common fixed boundaries when comparing surface densities for cloud populations within and between galaxies.