The X-factor in Galaxies: I. Dependence on Environment and Scale

TL;DR

This study models how the CO-to-H2 conversion factor, X_CO, varies with environmental factors like metallicity, density, and radiation, revealing its dependence on scale and implications for accurately estimating molecular gas in galaxies.

Contribution

The paper introduces a model combining small-scale simulations and galaxy formation data to predict X_CO's dependence on environment and scale, highlighting biases in observational estimates.

Findings

X_CO varies weakly with density at Milky Way conditions

X_CO strongly depends on metallicity, roughly as Z^{-0.5 to -0.8}

Neglecting environmental dependencies biases molecular gas estimates

Abstract

Characterizing the conversion factor between CO emission and column density of molecular hydrogen, X_CO, is crucial in studying the gaseous content of galaxies, its evolution, and relation to star formation. In most cases the conversion factor is assumed to be close to that of giant molecular clouds (GMCs) in the Milky Way, except possibly for mergers and star-bursting galaxies. However, there are physical grounds to expect that it should also depend on the gas metallicity, surface density, and strength of the interstellar radiation field. The XCO factor may also depend on the scale on which CO emission is averaged due to effects of limited resolution. We study the dependence of X_CO on gas properties and averaging scale using a model that is based on a combination of results of sub-pc scale magneto-hydrodynamic simulations and on the gas distribution from self-consistent cosmological simulations of galaxy formation. Our model predicts a value of X_CO that is consistent with the Galactic value for interstellar medium conditions typical for the Milky Way. For such conditions the predicted X_CO varies by only a factor of two for gas surfaced densities in the range \sim 50 - 500 M_sun / pc^2. However, the model also predicts that more generally on the scale of GMCs, X_CO is a strong function of metallicity, and depends on the column density and the interstellar UV flux. We show explicitly that neglecting these dependencies in observational estimates can strongly bias the inferred distribution of H2 column densities of molecular clouds to have a narrower and offset range compared to the true distribution. We find that when averaged on \sim kpc scales the X-factor depends only weakly on radiation field and column density, but is still a strong function of metallicity. The predicted metallicity dependence can be approximated as X_CO \sim Z^{-{\gamma}} with {\gamma} ~ 0.5 - 0.8.