SILCC-Zoom: H$_2$ and CO-dark gas in molecular clouds -- The impact of feedback and magnetic fields

D. Seifried, S. Haid, S. Walch, E. M. Borchert, T. G. Bisbas

Published 2019-06-03Version 1

We analyse the CO-dark molecular gas content of simulated molecular clouds from the SILCC-Zoom project. The simulations reach a resolution of 0.1 pc and include H$_2$ and CO formation, radiative stellar feedback and magnetic fields. CO-dark gas is found in regions with local visual extinctions $A_\text{V, 3D} \sim$ 0.2 -- 1.5, number densities of 3 -- 300 cm$^{-3}$ and gas temperatures of few 10 K -- 100 K. CO-bright gas is found at number densities above 300 cm$^{-3}$ and temperatures below 50 K. The CO-dark gas fractions range from 40% to 95% and scale inversely with the amount of well-shielded gas ($A_\text{V, 3D}$ $\gtrsim$ 1.5), which is smaller in magnetised molecular clouds. We show that the density, chemical abundances and $A_\text{V, 3D}$ along a given line-of-sight cannot be properly determined from projected quantities. As an example, we show that pixels with a projected visual extinction of $A_\text{V, 2D} \simeq$ 2.5 -- 5 can be both, CO-bright and CO-dark. By producing synthetic CO(1-0) emission maps of the simulations with RADMC-3D, we show that about 15 -- 65% of the H$_2$ is in regions with CO(1-0) emission below the detection limit. The simulated clouds have $X_\text{CO}$-factors around 1.5 $\times$ 10$^{20}$ cm$^{-2}$ (K km s$^{-1}$)$^{-1}$ with a spread of up to a factor $\sim$ 4, implying a similar uncertainty in the derived total H$_2$ masses and even worse results for individual pixels. Based on our results, we suggest a new approach to determine the H$_2$ mass, which relies on the availability of CO(1-0) emission and $A_\text{V, 2D}$ maps. It reduces the uncertainty of the clouds' overall H$_2$ mass to a factor of $\lesssim$ 1.8 and for individual pixels, i.e. on sub-pc scales, to a factor of $\lesssim$ 3.