9.5. A Cooling Flow Origin for the Cygnus A Activity?
As mentioned above, ROSAT data suggest a mass cooling flow rate of 250 M yr-1 for Cygnus A. Arguments pro and contra a cooling flow origin for the emission line gas in radio galaxies have been reviewed by Baum (1992). Also with reference to Baum et al. (1992) we here only mention that whereas a cooling flow origin for the gas cannot be excluded, Cygnus A is unusual in displaying high ionization gas and high radio luminosity, in combination with at most moderately ordered gas rotation (Tadhunter 1991, Stockton et al. 1994). This is at odds with the picture whereby the class of low luminosity/ionization radio galaxies are in cooling flow clusters and the high luminosity/ionization objects (such as Cygnus A) result from mergers, with the former displaying slow, ordered gas rotation and the latter fast, often large scale, rotation. However, while the optical spectrum of Cygnus A does show high ionization characteristics, it also has unusually strong low and intermediate ionization lines (e.g., Tadhunter et al. 1994). Also, Cygnus A may be a case where the observed gas kinematics is reduced by projection effects. Generally speaking, the issue of fueling the Cygnus A AGN (cooling flow or merger related) is still far from solved. There is little doubt that the high velocity gas observed both NW and E of the nucleus (Tadhunter 1991, Stockton et al. 1994) is accelerated through jet-ISM interaction. As for the origin of this gas, both a merger and a cooling flow can be envisaged.