Emission-line nebulae around central cluster galaxies in cooling flows

Abstract

New optical spectra of the emission-line nebulae at the centers of three cooling flows are presented. One, associated with the cluster A 496, is of class I in the line-ratio scheme of Heckman et al. (1989), and the second, in A2597, is of class II. Trends of line ratio and line luminosity with linewidth are found. It is shown 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.