Quasi-stellar objects as ?erenkov sources

If a beam of ultrarelativistic charges enters a gas cloud ?erenkov radiation may be produced. The condition for such radiation is given, and the ?erenkov power radiated per charge is compared to the synchrotron power. Intense ?erenkov pulses will excite emission lines via stimulated Raman scattering. Due to anomalous dispersion in the presence of ac Stark effect the ?erenkov field will be most intense at frequencies well red-shifted from atomic resonances, and this red-shift will be a feature of the stimulated Raman line emissions. The intensity ratios and anomalous red-shifts of the broad emission lines of QSOs can be explained. ?erenkov emission froma collimated beam will be highly directional. The emission evidently does not occur in BL Lacs, which are pure synchrotron emitters.