Gamma-rays from star-forming regions: from SNOBs to dark accelerators

Observational γ-ray astronomy was born some forty years ago, when small detectors were flown in satellites, following a decade of theoretical predictions of its potential to discover the origin of cosmic rays via the π-decay mechanism. The seventies were a golden era for γ-ray and cosmic-ray astrophysics, with the (re)discovery of the “diffuse shock acceleration” theory for cosmic rays, and the first CO and GeV γ-ray surveys of the galactic plane, verifying the importance of π-decay in the large-scale γ-ray emission of the Galaxy. But because of this strong galactic background, GeV γ-ray sources were hard to identify. The first such sources definitely identified were three pulsars, with a suggestion that supernova remnants interacting with molecular clouds in massive star-forming regions (“SNOBs”) were also γ-ray sources. Because of their improved sensitivity and spatial resolution, ground-based C̆erenkov telescopes, detecting γ-rays at > TeV energies, are now able to resolve molecular cloud-sized objects at a few kpc. SNOB-like objects like IC443 and W28 are detected at GeV and TeV energies, and show spatial evidence for cosmic ray interactions between an SNR shock wave and nearby molecular clouds, and subsequent πdecay. However, the spectral evidence does not clearly support this mechanism. We propose to use another tool for probing the interaction of the low-energy component of the putative local cosmic rays, in the form of enhanced ionization in TeV-bright molecular clouds, using millimeter observations. 1. Historical context: the “roaring seventies” of high-energy astro-