Emission-line strengths and cluster membership of radio galaxies

Samples of elliptical radio galaxies which are within or remote from rich clusters of galaxies were compiled by examining the fields of 190 radio galaxies with known redshifts on copies of Schmidt Sky Survey plates. The optical spectra of the radio galaxies were classified according to emission-line strengths. The two samples were compared with regard to emission-line strengths and other properties. An anti-correlation was found between the emission-line strengths and cluster membership of radio galaxies with luminosities at 408 MHz in the range 1024 to 1026 W Hz-1. Various interpretations of this result are discussed; these include gas removal mechanisms which may operate preferentially in clusters and the possibility that isolated radio galaxies are very massive and contain extensive gaseous atmospheres by gravitation. Optical emission lines may occur more frequently among cD and other centrally located radio galaxies in clusters than among less central radio galaxies in clusters. There is also a lack of emission lines among galaxies associated with old radio sources in clusters having steep low-frequency spectra. (author)