Physical properties of CO-dark molecular gas traced by C${}^{+}$

Abstract

Neither HI nor CO emission can reveal a significant quantity of so-called dark gas in the interstellar medium (ISM). It is considered that CO-dark molecular gas (DMG), the molecular gas with no or weak CO emission, dominates dark gas. We identified 36 DMG clouds with C${}^{+}$ emission (data from Galactic Observations of Terahertz C+ (GOT C+) project) and HINSA features. Based on uncertainty analysis, optical depth of HI $\ddot{I}„{}_{\mathrm{HI}}$ of 1 is a reasonable value for most clouds. With the assumption of $\ddot{I}„{}_{\mathrm{HI}}=1$, these clouds were characterized by excitation temperatures in a range of 20 K to 92 K with a median value of 55 K and volume densities in the range of $6.2\times 10^1$ cm${}^{-3}$ to $1.2\times 10^3$ cm${}^{-3}$ with a median value of $2.3\times 10^2$ cm${}^{-3}$. The fraction of DMG column density in the cloud ($f{}_{\mathrm{DMG}}$) decreases with increasing excitation temperature following an empirical relation $f{}_{\mathrm{DMG}}=-2.1\times 10^{-3}{\mathrm{T}}_{(}\mathrm{ex},\ddot{\mathrm{I}}{„}_{\mathrm{HI}}=1)$+1.0. The relation between $f{}_{\mathrm{DMG}}$ and total hydrogen column density $N_H$ is given by $f{}_{\mathrm{DMG}}$=$1.0-3.7\times 10^{20}/N_H$. The values of $f{}_{\mathrm{DMG}}$ in the clouds of low extinction group ($A{}_{\mathrm{V}}\le 2.7$ mag) are consistent with the results of the time-dependent, chemical evolutionary model at the age of ~ 10 Myr. Our empirical relation cannot be explained by the chemical evolutionary model for clouds in the high extinction group ($A{}_{\mathrm{V}}>2.7$ mag). Compared to clouds in the low extinction group ($A{}_{\mathrm{V}}\le 2.7$ mag), clouds in the high extinction group ($A{}_{\mathrm{V}}>2.7$ mag) have comparable volume densities but excitation temperatures that are 1.5 times lower. Moreover, CO abundances in clouds of the high extinction group ($A{}_{\mathrm{V}}>2.7$ mag) are $6.6\times 10^2$ times smaller than the canonical value in the Milky Way. #[Full version of abstract is shown in the text.]#

Accepted for publishing in Astronomy & Astrophysics. 13 pages, 8 figures