The C+ Universe

The interstellar medium (ISM) is central to galaxy evolution over cosmic time. Interstellar gas is known to exist as atomic (HI) clouds and star forming molecular clouds. Recent observations have revealed that half of interstellar gas is in so-called CO-dark molecular gas (hydrogen is H2 but carbon is C+). Atomic and CO-dark gas have so-far eluded detailed characterization because of lack of suitable tracer and the physical conditions of these components and their relationship to the molecular reservoir are largely unknown and their role in galactic ecology is wide open.The combination of sensitive THz heterodyne receiver arrays with a nimble telescope on SOFIA enables large scale, [CII] 158_m surveys of regions of massive star formation. This line is the main cooling line of the diffuse ISM and therefore a key diagnostic of interstellar gas energy balance. In addition, the high spectral resolution inherent to heterodyne techniques allows us to study in detail the kinematics of photodissociation regions, which separate ionized from molecular gas. I will present the first results of the [CII] 158_m square degree Orion Survey, its analysis and implications for large scale [CII] 158_m emission from galaxies.