Cerenkov radiation is radiation emitted when a charged particle travels through a medium at a speed greater than the velocity of light in the medium. This occurs when the refractive index of the medium is high. Cerenkov radiation is like the bow wave of a boat, or the shock wave of a supersonic airplane. Photons bunch up in front of the tachyon and they're radiated away at an angle determined by the speed of the tachyon. Cerenkov detectors are useful in atomic physics for determining the speed of particles moving through a medium.
Light slows as it passes through a medium.
That's what's responsible for optical effects. There's nothing mysterious about Cerenkov radiation in a medium. Now, there have been a few studies looking for Cerenkov radiation in a vacuum. This would indicated the reality of tachyons. Cerenkov radiation has never been detected in vacuum.