Magnetic Nanoparticles in the Interstellar Medium: Emission Spectrum and
  Polarization
  The presence of ferromagnetic or ferrimagnetic nanoparticles in the
interstellar medium would give rise to magnetic dipole radiation at microwave
and submm frequencies. Such grains may account for the strong mm-wavelength
emission observed from a number of low-metallicity galaxies, including the
Small Magellanic Cloud. We show how to calculate the absorption and scattering
cross sections for such grains, with particular attention to metallic Fe,
magnetite Fe3O4, and maghemite gamma-Fe2O3, all potentially present in the
interstellar medium. The rate of Davis-Greenstein alignment by magnetic
dissipation is also estimated. We determine the temperature of free-flying
magnetic grains heated by starlight and we calculate the polarization of the
magnetic dipole emission from both free-fliers and inclusions. For inclusions,
the magnetic dipole emission is expected to be polarized orthogonally relative
to the normal electric dipole radiation. Finally, we present self-consistent
dielectric functions for metallic Fe, magnetite Fe3O4, and maghemite
gamma-Fe2O3, enabling calculation of absorption and scattering cross sections
from microwave to X-ray wavelengths.
