Galaxy Interactions and the Radio Galaxy Phenomenon

Abstract

We have been studying powerful radio galaxies (which we define as having radio power at 178MHz, P 178 ≥ 5xl024 Watts/Hz for a cosmology of H o = 100 km sec-1 Mpc-1 and q o = 0) in order to determine their origins. There are two distinct classes of powerful radio galaxies (PRGs), the so called class A and B radio galaxies (Hine and Longair 1979; Heckman et al. 1986). The class A radio galaxies exhibit strong optical emission-lines (L H α > 1041erg/s), “edge-brightened” Fanaroff-Riley (1974) type II radio morphologies, dominate the radio luminosity function at L Radio > 1042 erg/s, and exhibit strong cosmological evolution. Two nearby examples are 3C 120 and NGC1275. The Class B radio galaxies typically have weak emission-lines or pure absorption line spectra, Fanaroff-Riley type I radio morphologies, dominate at L Radio ~1040-42 erg/s, and exhibit weak cosmological evolution. Some nearby prototypes are M84 and M87. If we hypothesize that these, like other active galaxies, may be the products of interactions or mergers we must account for their striking differences given their similar triggering mechanism. We have acquired (in collaboration with S. Baum, W. van Breugel, G. Illingworth, and G. Miley) a large body of data on roughly equal numbers of class A and B radio galaxies (in the redshift range 0.01 < z < 0.3) consisting of broad-band images (to study morphology, photometric structure, stellar content, dust, and the local environment), Narrow-band Hα images (to study the ISM), spectroscopy (to examine the stellar dynamics and the ISM), and radio maps. These data were principally acquired with the CTIO 4m, KPNO 4m, KPNO 2.1m telescopes, and the VLA.