  The GeV and TeV emission from M82 and NGC 253 observed by Fermi, HESS, and
VERITAS constrains the physics of cosmic rays (CRs) in these dense starbursts.
We argue that the gamma rays are predominantly hadronic in origin, as expected
by previous studies. The measured fluxes imply that pionic losses are efficient
for CR protons in both galaxies: we show that a fraction F_cal ~ 0.2 - 0.4 of
the energy injected in high energy primary CR protons is lost to inelastic
proton-proton collisions (pion production) before escape, producing gamma rays,
neutrinos, and secondary electrons and positrons. We discuss the factor ~2
uncertainties in this estimate, including supernova rate and leptonic
contributions to the GeV-TeV emission. We argue that gamma-ray data on ULIRGs
like Arp 220 can test whether M82 and NGC 253 are truly calorimetric, and we
present upper limits on Arp 220 from the Fermi data. We show that the observed
ratio of the GeV to GHz fluxes of the starbursts suggests that non-synchrotron
cooling processes are important for cooling the CR electron/positron
population. We briefly reconsider previous predictions in light of the
gamma-ray detections, including the starburst contribution to the gamma-ray
background and CR energy densities. Finally, as a guide for future studies, we
list the brightest star-forming galaxies on the sky and present updated
predictions for their gamma-ray and neutrino fluxes.
