Cosmic Rays and Magnetic Fields in the Core and Halo of the Starburst
  M82: Implications for Galactic Wind Physics
  Cosmic rays (CRs) and magnetic fields may be dynamically important in driving
large-scale galactic outflows from rapidly star-forming galaxies. We construct
two-dimensional axisymmetric models of the local starburst and super-wind
galaxy M82 using the CR propagation code GALPROP. Using prescribed gas density
and magnetic field distributions, wind profiles, CR injection rates, and
stellar radiation fields, we simultaneously fit both the integrated gamma-ray
emission and the spatially-resolved multi-frequency radio emission extended
along M82's minor axis. We explore the resulting constraints on the gas
density, magnetic field strength, CR energy density, and the assumed CR
advection profile. In accord with earlier one-zone studies, we generically find
low central CR pressures, strong secondary electron/positron production, and an
important role for relativistic bremsstrahlung losses in shaping the
synchrotron spectrum. We find that the relatively low central CR density
produces CR pressure gradients that are weak compared to gravity, strongly
limiting the role of CRs in driving M82's fast and mass-loaded galactic
outflow. Our models require strong magnetic fields and advection speeds of
order ~1000 km/s on kpc scales along the minor axis in order to reproduce the
extended radio emission. Degeneracies between the controlling physical
parameters of the model and caveats to these findings are discussed.
