Plasma suppression of synchrotron emission from radio-quiet infrared quasars

The authors calculate the synchrotron emission from a power-law distribution of relativistic electrons in a large-scale random magnetic field of strength B embedded in the thermal plasma of density n/sub e/. Due to the presence of the plasma, two major modifications of the classical vacuum theory of synchrotron emission are established: (1) synchrotron sources can be optically thick only in a rather small frequency range around the Razin-Tsytovich frequency v/sub R/=20 (n/sub e/w/cm/sup -3/) (B/G)/sup -1/ Hz, whereas at smaller and higher frequencies the sources are optically thin; (2) at frequencies beyond v/sub R/ the synchrotron intensity in a plasma behaves exactly the same way as in the vacuum case, whereas at frequencies below v/sub R/ the emitted intensity is exponentially reduced. It is argued that the established low-frequency cutoff at v/sub R/ due to the plasma suppression may be responsible for the measured sharp cutoff in the far infrared of 23 northern sky radio-quiet IRAS (Infrared Astronomy Satellite) infrared quasars. The cutoffs in the spectra of these radio-quiet quasars may be due to the exponential suppression at frequencies >