Radiative shock-wave theory. II - High-velocity shocks and thermal instabilities

The ionization structure and emission spectrum (optical and infrared) produced by low-density plasma cooling from initial temperatures as high as 10 exp 7.2 K are investigated. For initial temperatures higher than 10 exp 5.4 K, the optical spectra are dominated by emission from the low-temperature (about 10,000 K) photoionized regions that result from reabsorption of EUV ionizing flux emitted in the high-temperature zones. The predicted line intensities compare favorably with the emission filaments observed in M87 and NGC 1275, which are here assumed to result from cooling of an intracluster X-ray-emitting hot gas. A certain ambiguity makes it difficult in practice to choose between the latter models and pure photoionization models.