THE DARK MOLECULAR GAS

TLDR

Molecular cloud mass is usually inferred from CO line emission, yet a substantial fraction of H₂ lies outside the CO‐emitting region where it is shielded from UV yet undetectable in CO, making it a “dark” component traced only indirectly by gamma rays and dust continuum. The study models this dark H₂ component and shows that its mass fraction (~30 %) is nearly constant across clouds, insensitive to UV field, density, or cloud mass, provided the product of average hydrogen column and metallicity remains fixed. Additionally, the dark mass fraction rises as the average hydrogen column decreases, because more H₂ lies beyond the CO region.

Abstract

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 <T < 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 ) and insensitive to the incident ultraviolet radiation field strength, the internal density distribution, and the mass of the molecular cloud as long as , 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 , since relatively more molecular H2 material lies outside the CO region in this case.