Papers › Dependence of X_(CO) on metallicity, intensity, and spatial scale in a self-regulated...
Dependence of X_(CO) on metallicity, intensity, and spatial scale in a self-regulated interstellar medium
Chia-Yu Hu, Andreas Schruba, Amiel Sternberg, Ewine F. van Dishoeck
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We study the CO(1-0)-to-H₂ conversion factor (X_(CO)) and the line ratio of CO(2-1)-to-CO(1-0) (R₂₁) across a wide range of metallicity (0.1 ≤Z/Z_⊙ ≤3) in high-resolution (~0.2 pc) hydrodynamical simulations of a self-regulated multiphase interstellar medium. We construct synthetic CO emission maps via radiative transfer and systematically vary the "observational" beam size to quantify the scale dependence. We find that the kpc-scale X_(CO) can be over-estimated at low Z if assuming steady-state chemistry or assuming that the star-forming gas is H₂-dominated. On parsec scales, X_(CO) varies by orders of magnitude from place to place, primarily driven by the transition from atomic carbon to CO. The pc-scale X_(CO) drops to the Milky Way value of 2×10²⁰ cm⁻² (K km s⁻¹)⁻¹ once dust shielding becomes effective, independent of Z. The CO lines become increasingly optically thin at lower Z, leading to a higher R₂₁. Most cloud area is filled by diffuse gas with high X_(CO) and low R₂₁, while most CO emission originates from dense gas with low X_(CO) and high R₂₁. Adopting a constant X_(CO) strongly over- (under-)estimates H₂ in dense (diffuse) gas. The line intensity negatively (positively) correlates with X_(CO) (R₂₁) as it is a proxy of column density (volume density). On large scales, X_(CO) and R₂₁ are dictated by beam averaging, and they are naturally biased towards values in dense gas. Our predicted X_(CO) is a multivariate function of Z, line intensity, and beam size, which can be used to more accurately infer the H₂ mass.
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