QUASI-NORMAL MODES OF DIRTY BLACK-HOLES

Quasinormal mode (QNM) gravitational radiation from black holes is exp ected to be observed in a few years. A perturbative formula is derived for the shifts in both the real and the imaginary part of the QNM fre quencies away from those of an idealized isolated black hole. The form ulation provides a tool for understanding how the astrophysical enviro nment surrounding a black hole, e.g., a massive accretion disk, affect s the QNM spectrum of gravitational waves. We show, in a simple model, that the perturbed QNM spectrum can have interesting features.