ar X iv : a st ro - p h / 98 11 04 3 v 1 3 N ov 1 99 8 Microwave emission by dust : mechanisms , properties and prospects for ISM studies

I review my work with Bruce Draine on dust emissivity at microwave frequencies (3 cm-3 mm). This emissivity explains the recently detected " anomalous " component of the galactic foreground emission. Both small (a < 10 −7 cm) and large grains contribute to this emission. Small grains have electric dipole moments and emit while they rotate; the microwave emission of large grains is mostly due to magneto-dipole radiation. Most efficient magneto-dipole emitters are strongly magnetic, e.g. ferrimagnetic or ferromagnetic, materials. The relative role of the two mechanisms can be established through observations of microwave emissivity from dark clouds. New microwave window is a window of opportunity for interstellar studies. Magnetic fields inside dark clouds may be successfully studied via microwave polarization. Microwave emissivity constrains the abundance of strongly magnetic materials. For instance, the available data at 90 GHz indicate that not more than ∼ 5% of interstellar Fe is in the form of metallic iron grains or inclusions (e.g., in " GEMS "). Future missions, e.g. MAP and PLANCK, will bring a wealth of microwave data that can be successfully used to study ISM. Such a study would be appreciated by cosmologists who franticly try to remove all foregrounds from their data.