Speaker
Description
The CO-to-H$_2$ conversion factor $X_\text{CO}$ is commonly used to estimate the mass of molecular clouds in the interstellar medium (ISM), yet its applicability in galactic outflow remains poorly constrained. Starburst and AGN-host galaxies commonly exhibit high-velocity ($\gtrsim 100\ \mathrm{km\ s^{-1}}$) CO-emitting outflows. In these extreme environments, the density, temperature, and radiation field differ substantially from those of the ISM, , making the standard assumption of $X_\text{CO,\ MW}\approx 2\times 10^{20}\ \mathrm{cm^{-2}\ (K\ km\ s^{-1})^{-1}}$ potentially unreliable. We quantify $X_\text{CO}$ in galactic outflows using high-resolution GIZMO simulations of a $10^4\ \mathrm{K}$ cold cloud embedded in a hot, high-velocity wind, with a non-equilibrium chemistry network tracing CO formation and destruction. We find that the $X_\text{CO}$ exhibits systematic variations in both time and with distance from the cloud. Using a two-level approximation, we find that the majority of the CO-emitting gas is in non-LTE, and subsequently employ the radiative transfer code, pythonradex, to compute the non-LTE CO emission and derive $X_\text{CO}$. At late times, $X_\text{CO}$ rises to a factor of a few above the Milky Way value, providing a better estimate of the molecular mass in observed outflows.
| Participate the oral/poster presentation award competition | Yes |
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