Self-irradiated Cooling Condensations: The Source of the Optical Line Emission from Cooling Flows

The optical filaments seen at the centers of clusters of galaxies, which cannot be explained by standard photoionization or shock models, can be straightforwardly interpreted as self-irradiated cooling condensations. Fully developed (~ 10^4^ K) condensations embedded in thermally unstable 10^7^ K gas will be bathed in a powerful EUV/soft X-ray flux emanating from the surrounding condensing regions. Our models of the condensation/irradiation process give line ratios and luminosities similar to those observed. Because we assume that the condensations arise from linear perturbations, which grow significantly only when the local cooling time is less than the free- fall time, condensing regions of the type modeled here should arise only in cooling flows of central pressure >~ 10^-9^ ergs cm^-3^ and should extend only over the inner regions of the cluster.