The Parsec-Scale Relationship Between I_CO and A_V in Local Molecular Clouds

TL;DR

This study examines the relationship between CO emission and dust extinction in local molecular clouds, supporting the idea that CO emission depends mainly on shielding, and explores how this affects the CO-to-H2 conversion factor across different metallicities.

Contribution

It provides empirical measurements of the I_CO-A_V relation at parsec scales in 24 clouds and analyzes its implications for the CO-to-H2 conversion factor under varying metallicity conditions.

Findings

I_CO is uniform for a given A_V across clouds.

The I_CO-A_V relation agrees with previous studies and models.

Predicted X_CO varies with metallicity, with significant cloud-to-cloud differences at low metallicity.

Abstract

We measure the parsec-scale relationship between integrated CO intensity (I_CO) and visual extinction (A_V) in 24 local molecular clouds using maps of CO emission and dust optical depth from Planck. This relationship informs our understanding of CO emission across environments, but clean Milky Way measurements remain scarce. We find uniform I_CO for a given A_V, with the results bracketed by previous studies of the Pipe and Perseus clouds. Our measured I_CO-A_V relation broadly agrees with the standard Galactic CO-to-H2 conversion factor, the relation found for the Magellanic clouds at coarser resolution, and numerical simulations by Glover & Clark (2016). This supports the idea that CO emission primarily depends on shielding, which protects molecules from dissociating radiation. Evidence for CO saturation at high A_V and a threshold for CO emission at low A_V varies remains uncertain due to insufficient resolution and ambiguities in background subtraction. Resolution of order 0.1 pc may be required to measure these features. We use this I_CO-AV relation to predict how the CO-to-H2 conversion factor (X_CO) would change if the Solar Neighborhood clouds had different dust-to-gas ratio (metallicity). The calculations highlight the need for improved observations of the CO emission threshold and HI shielding layer depth. They are also sensitive to the shape of the column density distribution. Because local clouds collectively show a self-similar distribution, we predict a shallow metallicity dependence for X_CO down to a few tenths of solar metallicity. However, our calculations also imply dramatic variations in cloud-to-cloud X_CO at subsolar metallicity.