Dark Gas in the Translucent Cloud MBM 12

Gamma-ray studies of nearby molecular clouds show some residual emission after removing emission derived from spatial maps of HI and CO. This residual emission is called “dark gas” and it represents molecular hydrogen not traced by CO. We study the gamma-ray emission from MBM 12, a nearby translucent cloud. These clouds have very low column density which allows UV radiation from the interstellar radiation field to penetrate through the entire cloud. The UV irradiation creates large photodissociation regions in the cloud, where a significant amount of CO is dissociated into atomic carbon. The neutral and ionized atomic carbon should trace the molecular hydrogen in these regions. MBM 12 is free of known sources of high energy cosmic rays, such as OB associations or supernova remnants, and it is both close and extended enough that we expect it to be resolvable in gamma-rays with the Fermi LAT. This makes it an ideal laboratory to identify whether other molecular tracers, such as atomic carbon, can trace the dark gas. We compare the gamma-ray emission with spatial maps of HI, CO, dust, CI, and CII to try to identify the source of the dark gas.