The Role of Magnetic Fields in Starburst Galaxies as Revealed by OH
  Megamasers
  We present estimates of magnetic field strengths in the interstellar media of
starburst galaxies derived from measurements of Zeeman splitting associated
with OH megamasers. The results for eight galaxies with Zeeman detections
suggest that the magnetic energy density in the interstellar medium of
starburst galaxies is comparable to their hydrostatic gas pressure, as in the
Milky Way. We discuss the significant uncertainties in this conclusion, and
possible measurements that could reduce these uncertainties. We also compare
the Zeeman splitting derived magnetic field estimates to magnetic field
strengths estimated using synchrotron fluxes and assuming that the magnetic
field and cosmic rays have comparable energy densities, known as the "minimum
energy" argument. We find that the minimum energy argument systematically
underestimates magnetic fields in starburst galaxies, and that the conditions
that would be required to produce agreement between the minimum energy estimate
and the Zeeman derived estimate of interstellar medium magnetic fields are
implausible. The conclusion that magnetic fields in starburst galaxies exceed
the minimum energy magnetic fields is consistent with starburst galaxies
adhering to the linearity of the FIR-radio correlation.
