"Slow group velocity and Cherenkov radiation"

We study theoretically the effect of ultraslow group velocities on the emission of Vavilov-Cherenkov ra- diation in a coherently driven medium. We show that in this case the aperture of the group cone on which the intensity of the radiation peaks is much smaller than that of the usual wave cone associated with the Cherenkov coherence condition. As a specific example, we consider a coherently driven ultracold atomic gas where such singular behavior may be observed.