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_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 _R=20 (n_ew/cm^-3) (B /G)^-1 Hz, whereas at smaller and higher frequencies the sources are optically thin; (2) at frequencies beyond v_R the synchrotron intensity in a plasma behaves exactly the same way as in the vacuum case, whereas at frequencies below v_R the emitted intensity is exponentially reduced. It is argued that the established low-frequency cutoff at v_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 ⩽2×10¹² Hz ( n_e/10⁸ cm^-3) (B/10 ^-3 G)^-1