9.4. A Merger Origin for the Cygnus A Galaxy?
Whereas merging is most likely an important mechanism triggering nuclear activity in galaxies (e.g., Heckman et al. 1986, Stockton 1990), there is only limited direct evidence that Cygnus A has gone through such a process. The well established r1/4 surface brightness profile by itself does not exclude a recent merger (e.g. Schweizer 1982), but that fact combined with the absence of large scale tidal features and host distortion provides little evidence for a recent merger scenario. The best evidence that a merger may have taken place in Cygnus A is in our view the detection by Stockton et al. (1994) of the secondary 2.2µ peak close to the bright nuclear emission line cloud (see Fig. 13) and of its peculiar nuclear gas kinematics (cf. Hernquist and Barnes 1991). Although the surplus of young stars in the nuclear regions does not require a merger origin, it is certainly consistent with it (Hernquist and Weil 1992).
Mazzarella et al. (1993) discuss how the activity in radio galaxies may be fundamentally related to starbursts, with both being triggered through a recent merger. Supporting evidence comes from the fact that many radio galaxies show peculiar morphologies, such as double nuclei and kpc-scale tails or shells, indicative of non-relaxed structures as expected in recent merger events (Heckman et al. 1986). Mazzarella et al. (1993) find that several nearby radio galaxies show CO emission, with implied H2 masses between 5 x 109 and 5 x 1010 M, or one to seven times the H2 mass of the Milky Way. For comparison, radio quiet far-infrared selected elliptical galaxies have H2 masses 2 to 3 orders of magnitude lower than the Milky Way. This suggests rich supplies of molecular gas in radio galaxies. Moreover, radio galaxies show a similar correlation between LFIR and M(H2) as starbursts, and the molecular mass-to-IR luminosity ratio for radio galaxies is in the range typical of nuclear starburst galaxies (15 to 100 L / M). Mazzarella et al. (1993) hypothesize that `radio galaxies originate in colliding disk galaxies which evolve into gas-rich, peculiar E/S0 galaxies during the merger process.' Again, Cygnus A was not detected in CO emission, with the limit being at the low end for emission seen from nearby radio galaxies. Mazzarella et al. therefore speculate that Cygnus A may represent a dynamically old merger, having already processed a substantial gas mass of its parent disk galaxies. A larger, more homogeneous radio galaxy CO data base is however needed to address this issue further.
It should be kept in mind that the estimated age of the radio source is 107 years. It will be of interest to model a possible Cygnus A merging scenario taking the age of the radio source into account.