8. THE X_RAY CLUSTER

8.1. Large Scale Gas Distribution

Cygnus A was first detected as an X-ray source by Giacconi et al. (1972) using the Uhuru satellite (see also Longair and Willmore 1974). Follow-up observations with the HEAO1 satellite by Fabianno et al. (1979) found that the source is spatially extended. They suggested that the emission is due to an atmosphere of hot gas with a temperature of 7 x 10⁷ K, such as had been seen for the Virgo and Perseus clusters (see also Brinkman et al. 1977). This interpretation of a hot cluster halo for Cygnus A was confirmed by Arnaud et al. (1984) using the Einstein observatory. Their IPC image shows a very extended cluster gas distribution, with a radius 500 h^-1 kpc, a density distribution radius^-1, and a total mass of 10¹⁴ M, while the Einstein HRI image revealed a dense core of emission, within which the radio source is embedded. The most recent treatment of the Cygnus A cluster gas emission is by Reynolds and Fabian (1995). Using ROSAT PSPC and HRI data, they show that the cooling time for the gas within 90 h^-1 kpc is less than the Hubble time. They calculate a `cooling flow' rate of 250 M yr^-1 for the dense cluster gas.

Spectral observations of Cygnus A with the Ginga satellite have refined the parameters for the cluster gas (Ueno et al. 1994). They find the total emission from Cygnus A is best fit by a two-component model including roughly comparable contributions from a highly absorbed power-law component (presumably the active nucleus), and from thermal cluster gas at T = 8.5 x 10⁷ K. The absorption corrected luminosity of the cluster between 2 keV and 10 keV is 6 x 10⁴⁴ h^-2 ergs sec^-1 (using a Galactic HI column density of 2.4 x 10²¹ cm^-2). They also detect iron line emission at rest-frame energy 6.9 keV, with a best-fit abundance of 0.3 x solar. These conclusions have been confirmed through recent observations with ASCA (Arnaud et al. 1996). Lastly, the spatially resolving ROSAT PSPC observations of Cygnus A by Reynolds and Fabian (1995) suggest a temperature gradient to the cluster center, with the temperature decreasing from 8 x 10⁷ K at radii 150 h^-1 kpc, to 3 x 10⁷ K in the inner 50 h^-1 kpc. Such a temperature decrease is qualitatively consistent with the cooling flow scenario.

The origin and evolution of hot cluster atmospheres is reviewed in detail in Sarazin (1986, 1988), and we refer the reader to these reviews for further details on this subject. We simply point out a few of the curiosities of the Cygnus A cluster. First is the anisotropic distribution of the cluster gas on large-scales. The cluster shows a long `tail' extending about 500 h^-1 kpc to the northwest. Such a morphology implies that the system has not reached final dynamical equilibrium, perhaps indicating a recent cluster merger (time-scale cluster sound crossing time 500 Myr). And second is that optical imaging of Cygnus A suggests that the Cygnus A cluster is poor in galaxies, with only four galaxies identified other than Cygnus A itself (Spinrad and Stauffer 1982). This contrasts sharply with the rich gaseous environment of the Cygnus A cluster, and leads to questions concerning the origin and heating of the cluster gas. However, the Cygnus A optical field is very confused, and images with sub-arc-second resolution are required for proper determination of the true galaxy content of the Cygnus A cluster.