The mass of molecular gas in an interstellar cloud is often measured using line emission from low rotational levels of CO, which are sensitive to the CO mass, and then scaling to the assumed molecular hydrogen H2 mass. However, a significant H2 mass may lie outside the CO region, in the outer regions of the molecular cloud where the gas-phase carbon resides in C or C+. Here, H2 self-shields or is shielded by dust from UV photodissociation, whereas CO is photodissociated. This H2 gas is “dark” in molecular transitions because of the absence of CO and other trace molecules, and because H2 emits so weakly at temperatures 10 K < 100 K typical of this molecular component. This component has been indirectly observed through other tracers of mass such as gamma rays produced in cosmic-ray collisions with the gas and far-infrared/submillimeter wavelength dust continuum radiation.

In this paper, we theoretically model this dark mass and find that the fraction of the molecular mass in this dark component is remarkably constant (∼0.3 for average visual extinction through the cloud A¯V8) and insensitive to the incident ultraviolet radiation field strength, the internal density distribution, and the mass of the molecular cloud as long as A¯V , or equivalently, the product of the average hydrogen nucleus column and the metallicity through the cloud, is constant. We also find that the dark mass fraction increases with decreasing A¯V , since relatively more molecular H2 material lies outside the CO region in this case.

Various observations have indicated that a substantial amount of interstellar gas exists in the form of molecular hydrogen (H2) along with ionized carbon (C+), but little or no carbon monoxide (CO). The total mass in molecular hydrogen has been estimated from gamma-ray observations from COS-B (Bloemen et al. 1986) and the Energetic Gamma-Ray Experiment Telescope (EGRET; Strong & Mattox 1996), and analysis of this data showed more gas mass than can be accounted for in H i and CO alone (Grenier et al. 2005). In addition, the dust column density maps of the Galaxy from DIRBE, and maps of the Two Micron All Sky Survey (2MASS) J − K extinction show additional gas not seen in H i or CO (Grenier et al. 2005) and is presumably molecular hydrogen.

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Journal of Molecular Sciences
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