The interrelationship between cosmic dust and the microwave background

Abstract

Attention is given to the radiation of microwaves by charged dust in space. Presently-used particle distributions do not restrict the presence in space of large numbers of small (r<10⁻⁶ cm) silicate grains, but it is shown that such densities (ϱ≈10⁻²⁵–10⁻²⁶ g cm⁻³) of small grains would produce a microwave background with an energy density of the same order of magnitude as the energy density of the (presumed) cosmological 3 K background. Limits set by the isotropy of the latter are: ϱ(HI clouds)≲10⁻²⁶, ϱ(Galactic plane)≲10⁻³⁰, ϱ(Halo)≲10⁻³², ϱ(Local Group)≲10⁻³⁴ g cm⁻³. These limits imply that either there is a cutoff in particle distributions atr≃10⁻⁶ cm, or that the density of silicate grains in space has been generally overestimated, or that cosmic rays have broken up a lot of grains so that they now form a population of grains of very small size (≃10⁻⁷ cm) which are difficult to detect by conventional methods. One way to look for the latter population is by studying expected distortions of the 3 K spectrum to the short wavelength side of the portion hitherto observed (grains may have a size distribution able to give an approximate black-body curve for radiation from larger grains of 10⁻⁶ cm size), and by testing the effective energy density of the 3 K field in other galaxies.