Multicolor surface photometry of powerful radio galaxies. II - Morphology and stellar content

We present results on the morphology, photometric structure, and colors of 72 powerful radio galaxies (P_178_ >= 5 X 10^24^ W Hz^-1^), and discuss the implications these finds have for theories concerning the origin of activity in galaxies. Sample selection, observational procedures, and data reduction techniques for this study are detailed in a recent paper by Smith and Heckman. We report three major conclusions regarding the nature of powerful radio galaxies (PRGs). The first is that galaxy interactions/mergers play an important role in the PRG phenomenon. We find that over 50% of the sample galaxies display optical morphological deviations from elliptical symmetry at high levels of surface brightness (μ_v_ <= 25 mag arsec^-2^). More specifically, we find that about half of the PRGs with strong optical emission line spectra (SE PRGs)--but only 7% of the PRGs with weak or no emission lines (WE/ABS PRGs) exhibit peculiar optical morphologies (tails, fans, bridges, shells, and dust lanes). Narrow or sharp features such as these may be explained by galaxy interactions involving at least one gas-rich, dynamically cold galaxy (i.e., a disk galaxy). We find that about 20% of our galaxies have a second nucleus less than 10 kpc in projection from the main nucleus (30% of the WE/ABS PRGs and only 10% of the SE PRGs). The fraction of PRGs in a "common envelope" with neighboring galaxies is even larger (40% for the WE/ABS PRGs and 20% for the SE galaxies). Photometric investigations of the sample indicate that the surface brightness profiles for the WE/ABS galaxies are typically shallower in slope than normal radio-quiet elliptical galaxies, but similar to brightest cluster galaxies. Surface brightness profiles for the SE galaxies are more diverse in form. The second major conclusion is that the SE PRGs have unusually blue average colors relative to giant elliptical galaxies (by ~0.2 mag in (B- V)]. It is important to note that these blue colors are spatially extended and not merely due to light from a bright nucleus or extended emission-line gas (both of these contributions having been explicitly removed), and as such they must reflect the stellar content of the PRG. Also the SE PRGs often exhibit strong spatial variations in color. We hypothesize that the unusual colors and color variations are related to merger-induced star formation. In contrast, the WE/ABS PRGs have normal colors and color gradients for giant elliptical galaxies. Third, we note the strong similarity between the WE/ABS PRGs and brightest cluster galaxies. They have similar absolute magnitudes, incidence rates of double/multiple nuclei, colors, and surface brightness profiles.