Suprathermal grains: On intergalactic magnetic fields

Abstract

Charged dust grains of radiia≃3×10−6∼3×10−5 cm may be driven out of the galaxy due to radiation pressure of starlight. Once clear of the main gas-dust layer, dust grains may then escape into intergalactic space. Such grains are virtually indestructible-being evaporated only during galaxy formation. The dust grains, once injected into the intergalactic medium, may acquire suprathermal energy, thus ‘suprathermal grains’ in collision with magnetized cloud by the Fermi process. In order to attain relativistic energy, suprathermal grains have to move in and out (‘scattering’) of the magnetic field of the medium. It is now well established that high energy cosmic rays are of the order 1020 eV or more. We have speculated that these high energy (>-1018 eV) cosmic ray particles are charged dust grains, of intergalactic origin. This is possible only if there exists a magnetic field in the intergalactic medium.