Dependence of $X_{\rm CO}$ on metallicity, intensity, and spatial scale in a self-regulated interstellar medium

Abstract

We study the CO(1-0)-to-H 2 conversion factor (X CO ) and the line ratio of CO(1-0)-to-CO(2-1) (R 21 ) 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 H 2 -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 × 10 20 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 R 21 . Most cloud area is filled by diffuse gas with high X CO and low R 21 , while most CO emission originates from dense gas with low X CO and high R 21 . Adopting a constant X CO strongly over-(under-)estimates H 2 in dense (diffuse) gas. The line intensity negatively (positively) correlates with X CO (R 21 ) as it is a proxy of column density (volume density). On large scales, X CO and R 21 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 H 2 mass.

Slow Star Formation in the Milky Way: Theory Meets Observations

“…which has a substantially less steep dependence on Z than is often adopted in extragalactic observations. A similar dependence (Z −0.7 ) was found in simulations including time-dependent effects by Hu et al (2022). Improved comparison to the actual values of N H 2 in the Gong et al (2020) simulations is provided by a formula that also accounts for the intensity (W CO ) of the emission and the spatial resolution of the survey (r beam ).…”

“…As can be seen from Figure 1, we are mildly extrapolating those results in the very outer Galaxy and, more importantly, in the inner Galaxy. Extending the simulations over a wider range of Z would be very valuable (see, e.g., Hu et al 2022).…”

Slow Star Formation in the Milky Way: Theory Meets Observations

“…which has a substantially less steep dependence on Z than is often adopted in extragalactic observations. A similar dependence (Z −0.7 ) was found in simulations including time dependent effects by Hu et al (2022). Improved comparison to the actual values of N H2 in the Gong et al (2020) simulations is provided by a formula that also accounts for the intensity (W CO ) of the emission and the spatial resolution of the survey (r beam ).…”

What Causes The Formation of Disks and End of Bursty Star Formation?

“…Similarly, if we change the density threshold for star formation it does nothing to alter the global balance: forcing a low star formation rate in dense gas merely means more gas will "pile up" at high densities before feedback can selfregulate it, and gas at those high densities will get denser and denser before being exhausted or disrupted, so the details of the phase distribution of gas (e.g. amount of gas at the highest densities relative to that at intermediate densities) will change, but to leading order the global feedback properties are unchanged (again, this is shown in much more detail in previous studies, see Hopkins et al , 2013bHu et al 2022). Changing the "form" of feedback, e.g.…”