Magnetic Nanoparticles in the Interstellar Medium: Emission Spectrum and Polarization

TL;DR

This paper investigates the emission and polarization properties of ferromagnetic and ferrimagnetic nanoparticles in the interstellar medium, providing models for their absorption, scattering, and alignment, which could explain observed microwave emissions in certain galaxies.

Contribution

It introduces detailed calculations of absorption, scattering, and polarization for magnetic nanoparticles, including their dielectric functions and alignment mechanisms, advancing understanding of interstellar magnetic dust.

Findings

Magnetic nanoparticles can produce significant microwave emission.

Polarization of magnetic dipole emission is orthogonal to electric dipole radiation.

Self-consistent dielectric functions enable accurate cross section calculations.

Abstract

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.