SILCC-Zoom: H2 and CO-dark gas in molecular clouds – the impact of feedback and magnetic fields

Abstract

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 V,3D ∼ 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 V,3D 1.5), which is smaller in magnetised molecular clouds. We show that the density, chemical abundances and A 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 V,2D ≃ 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 CO -factors around 1.5 × 10 20 cm −2 (K km s −1 ) −1 with a spread of up to a factor ∼ 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 V,2D maps. It reduces the uncertainty of the clouds' overall H 2 mass to a factor of 1.8 and for individual pixels, i.e. on sub-pc scales, to a factor of 3.

H I filaments are cold and associated with dark molecular gas

“…CO-dark H 2 gas is depleted in CO-rich regions since the CO-dark part lies outside the dominant CO region (Wolfire et al 2010). Numerical simulations by Seifried et al (2020) show that the dark gas fraction scales inversely with the amount of well-shielded gas for E(B−V) > ∼ 0.5. This is the range where we find a considerable drop in the dark gas fraction, on average f N H2 < ∼ 0.4 (Fig.…”

“…A significant part of the H 2 appears to be distributed around dense molecular gas cores; we refer to the model proposed by Wolfire et al (2010) and Fig. 1 of Seifried et al (2020). We apply a factor X CO = 4.0 × 10 20 cm −2 (K km s −1 ) −1 to calculate the CO-bright H 2 .…”

Tracing the total molecular gas in galaxies: [CII] and the CO-dark gas

“…Comparison of Herschel [C ii] velocity profiles to H i emission (Langer et al 2010(Langer et al , 2014Pineda et al 2013;Tang et al 2016) as well as observations comparing dust, CO (1−0), and H i (Planck Collaboration XIX 2011; Reach et al 2017) confirm that this CO-dark gas reservoir can be an important component in our Galaxy, and comparable to that traced by CO (1−0) alone. Recent hydrodynamical simulations with radiative transfer and analytic theory also demonstrate the presence of this dark gas reservoir in the Galaxy that should be traceable via [C ii] (Smith et al 2014;Offner et al 2014;Glover & Clark 2016;Nordon & Sternberg 2016;Gong et al 2018;Franeck et al 2018;Seifried et al 2020) and is often associated with spiral arms in disc galaxies. While optically thick H i can, in principle, be the source of the CO-dark gas, there is no compelling evidence that it contributes significantly to the dark neutral gas (e.g.…”

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

“…On the other hand, large-scale galaxy simulations with resolution 50 pc (e.g., Narayanan et al 2012;Feldmann et al 2012;Richings & Schaye 2016) cannot resolve the structures of molecular clouds and therefore rely on over-simplified assumptions on the subgrid gas distribution. Recently, it has become feasible to simulate a kpc-scale ISM patch and follow star formation and stellar feedback self-consistently, establishing a more realistic, self-regulated system with pc-scale resolution such that the ISM structure and stellar feedback are resolved without resorting to sub-grid models (Gatto et al 2017;Seifried et al 2017;Gong et al 2018;Seifried et al 2020;Smith et al 2020). However, most of these studies focus on solar-metallicity, solar-neighborhood conditions, and the only exception (Gong et al 2020) studied X CO with a limited range of 0.5 ≤ Z ≤ 2, which still does not cover the "low-metallicity" regime.…”

“…In Hu et al (2021) (hereafter HSvD21), we presented a suite of hydrodynamical simulations of a (1 kpc) 2 ISM patch with an unprecedented dynamical range spatially and temporally, running for 500 Myr and reaching sub-parsec (∼0.2 pc) spatial resolution (which has previously only been achieved with the "zoom-in" techniques and was limited to a few Myrs, e.g., Seifried et al 2020;Smith et al 2020). The long simulated time leads to a large sample of clouds at different evolutionary stages, while the sub-parsec resolution is crucial for resolving the compact CO-bright cores at low metallicities.…”