2016A&A...593A..42T

Context. 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. Determination of physical properties of DMG is critical for understandingISM evolution. Previous studies of DMG in the Galactic plane are based on assumptions of excitation temperature and volume density.Independent measurements of temperature and volume density are necessary.
Aims. We intend to characterize physical properties of DMG in the Galactic plane based on C⁺ data from the Herschel open time key program, namely Galactic Observations of Terahertz C+ (GOT C+) and HI narrow self-absorption (HINSA) data from international HI 21cm Galactic plane surveys.
Methods. We identified DMG clouds with HINSA features by comparing HI, C⁺, and CO spectra. We derived the HI excitation temperature and HI column density through spectral analysis of HINSA features. The HI volume density was determined by utilizing the on-the-sky dimension of the cold foreground HI cloud under the assumption of axial symmetry. The column and volume density of H₂ were derived through excitation analysis of C⁺ emission. The derived parameters were then compared with a chemical evolutionary model.
Results. We identified 36 DMG clouds with HINSA features. Based on uncertainty analysis, optical depth of HIτ_HI of 1 is a reasonable value for most clouds. With the assumption of τ_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 6.2x10¹cm^–3 to 1.2x10³cm^–3 with a median value of 2.3x10²cm^–3. The fraction of DMG column density in the cloud (f_DMG) decreases with increasing excitation temperature following an empirical relation f_DMG=-2.1x10^–3T_ex,(τ_HI=1)+1.0. The relation between f_DMG and total hydrogen column density N_H is given by f_DMG=1.0-3.7x10²⁰/N_H. We divided the clouds into a high extinction group and low extinction group with the dividing threshold being total hydrogen column density N_H of 5.0x10²¹cm^–2 (A_V=2.7mag). The values of f_DMG in the low extinction group (A_V≥2.7mag) are consistent with the results of the time-dependent, chemical evolutionary model at the age of ∼10Myr. Our empirical relation cannot be explained by the chemical evolutionary model for clouds in the high extinction group (A_V>2.7mag). Compared to clouds in the low extinction group (A_V≥2.7mag), clouds in the high extinction group (A_V>2.7mag) have comparable volume densities but excitation temperatures that are 1.5 times lower. Moreover, CO abundances in clouds of the high extinction group (A_V>2.7mag) are 6.6x10² times smaller than the canonical value in the Milky Way.
Conclusions. The molecular gas seems to be the dominate component in these clouds. The high percentage of DMG in clouds of the high extinction group (A_V > 2.7 mag) may support the idea that molecular clouds are forming from pre-existing molecular gas, i.e., a cold gas with a high H₂ content but that contains a little or no CO content.