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Gamma-Ray Dominated Regions: Extending the Reach of Cosmic Ray Ionization in Starburst Environments

Cosmic rays are appealing as a source of ionization in starburst galaxies because of the great columns they can penetrate, but in the densest regions of starbursts, they may be stopped by pion production and ionization energy losses. I argue that gamma rays are the source of ionization in the deepest molecular clouds of dense starbursts, creating Gamma-Ray Dominated Regions (GRDRs). Gamma rays are not deflected by magnetic fields, have a luminosity up to ~1/3 that of the injected cosmic rays, and can easily penetrate column depths of ~100 g/cm^2 before being attenuated by gamma-Z pair production. The ionization rates of GRDRs, <~10^-16 s^-1, are much smaller than in cosmic ray dominated regions, but in the most extreme starbursts, they may still reach values comparable to those in Milky Way molecular clouds. The gas temperatures in GRDRs could be likewise low, <~10 K if there is no additional heating from dust or turbulence, while at high densities, the kinetic temperature will approach the dust temperature. The ratio of ambipolar diffusion time to free-fall time inside GRDRs in dense starbursts is expected to be similar to those in Milky Way cores, suggesting star-formation can proceed normally in them. The high columns of GRDRs may be opaque even to millimeter wavelengths, complicating direct studies of them, but I argue that they could appear as molecular line shadows in nearby starbursts with ALMA. Since GRDRs are cold, their Jeans masses are not large, so that star-formation in GRDRs may have a normal or even bottom-heavy initial mass function.