X-Ray Emission from Star-Forming Galaxies - Signatures of Cosmic Rays
  and Magnetic Fields
  The evolution of magnetic fields in galaxies is still an open problem in
astrophysics. In nearby galaxies the far-infrared-radio correlation indicates
the coupling between magnetic fields and star formation. The correlation arises
from the synchrotron emission of cosmic ray electrons traveling through the
interstellar magnetic fields. However, with an increase of the interstellar
radiation field (ISRF), inverse Compton scattering becomes the dominant energy
loss mechanism of cosmic ray electrons with a typical emission frequency in the
X-ray regime. The ISRF depends on the one hand on the star formation rate and
becomes stronger in starburst galaxies, and on the other hand increases with
redshift due to the evolution of the cosmic microwave background. With a model
for the star formation rate of galaxies, the ISRF, and the cosmic ray spectrum,
we can calculate the expected X-ray luminosity resulting from the inverse
Compton emission. Except for galaxies with an active galactic nucleus the main
additional contribution to the X-ray luminosity comes from X-ray binaries. We
estimate this contribution with an analytical model as well as with an
observational relation, and compare it to the pure inverse Compton luminosity.
Using data from the Chandra Deep Field Survey and far-infrared observations
from ALMA we then determine upper limits for the cosmic ray energy. Assuming
that the magnetic energy in a galaxy is in equipartition with the energy
density of the cosmic rays, we obtain upper limits for the magnetic field
strength. Our results suggest that the mean magnetic energy of young galaxies
is similar to the one in local galaxies. This points toward an early generation
of galactic magnetic fields, which is in agreement with current dynamo
evolution models.
