Emission-line nebulae around central cluster galaxies in cooling flows

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Abstract
We present new optical spectra of the emission-line nebulae at the centres of three cooling flows. One, associated with the cluster A496, is of class I in the line-ratio scheme of Heckman et al., and the second, in A2597, is of class II. The third, in Sérsic 159-03, shows line ratios in the nucleus region which lie between those of classes I and II; a filament at 20 kpc radius shows ratios of extreme class II. We find trends of line ratio and line luminosity with linewidth. Previous work on the source of ionization of these nebulae has been inconclusive, since any mechanism is required to be distributed throughout the nebula. We show here that the self-absorbed irradiation of small shreds of cold gas in mixing layers on the surfaces of dense clouds successfully reproduces the observed class I line ratios. The key ingredient is a hard ionizing spectrum. The main source of energy in this case is the thermal energy of the turbulent hot gas which mixes with the cold gas. The class II nebulae are identified with systems in which there is an additional component due to shocks between dense cold molecular clouds in the flow. The most optically luminous systems must be transient and probably occur when the gas in the core of a flow is violently disturbed by a cluster merger or a powerful radio source.

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