9.3. Infrared, Submm, and MM Emission

The line and continuum reddening mentioned above does not come as a surprise, since dust is abundant in the Cygnus A galaxy. IRAS data for Cygnus A and other radio galaxies were analyzed by Golombek et al. (1988), Knapp et al. (1990), and most recently Hes et al. (1995). The last authors plot 60µ luminosities as function of 178 MHz luminosities for the IRAS detected 3CR radio sources. Comparison of this plot (Fig. 11) with Fig. 1 suggests that although its 60µ luminosity is 2 x 10¹¹ L (within, but at the low end of the range for 3CR quasars), Cygnus A is anomalous in its radio rather than in its far-infrared emission. Barthel and Arnaud (1995) attribute the anomalously high radio luminosity of Cygnus A to comparatively strong radiation losses in the extended X-ray halo.

Figure 11. Luminosities L of all 3CR radio galaxies and quasars detected at 60µ versus total radio power at 178 MHz, reproduced from Hes et al. (1995). Filled circles represent FRII quasars, open circles FRII radio galaxies, open triangles FRI radio galaxies. The marked sources are the starburst galaxy M 82 (3C 231) and Cygnus A (3C 405).

Dealing with the nuclear extinction in Cygnus A, Djorgovski et al. (1991) present the overall spectral energy distribution from its central component. It can be seen that most of the far-infrared emission must be thermal, and Djorgovski et al. (1991) fit a dust temperature of 75K. Depending on the poorly constrained submm spectrum, the nonthermal far-infrared contribution can be estimated to be between a few and ten per cent.

Searches have also been made for CO emission from Cygnus A (Mirabel et al. 1989, Mazzarella et al. 1993, O'Dea et al. 1994, McNamara and Jaffe 1994), all unsuccessful. The most sensitive limits are by Mazzarella et al. (1993), who determine an upper limit to the mass in cold H₂ (using standard conversion factors) of 3 x 10⁹ M. This upper limit to the cold molecular gas mass for Cygnus A is at the low end for masses seen in other radio galaxies (Mazzarella et al. 1993).

Considering the hot molecular gas, Ward et al. (1991) measure an unusually strong H₂ = 1-0 S(1) emission line from Cygnus A, with a luminosity 1.0 x 10⁴¹ erg sec^-1, which brings Cygnus A close to classical star forming galaxies such as Arp 220 and NGC 6240. The implied mass of hot molecular gas leads to a mass ratio upper limit of hot/cold H₂ which is however not unusual. Ward et al. (1991) speculate that both X-ray heating and shock heating may be responsible for the H₂ excitation.