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For refcode 1997ApJ...476..489Z:
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1997ApJ...476..489Z The Megaparsec Environments of Radio Galaxies ESTHER L. ZIRBEL Department of Astronomy, Yale University; and Space Telescope Science Institute; 2 ezirbel@haverford.edu Received 1995 February 8; accepted 1996 September 4 ABSTRACT This paper presents an analysis of the Mpc environments of powerful radio galaxies (with z < 0.5 and radio powers P_408 MHz_ > 10^26^ W Hz^- 1^, using H_0_ = 50 km s^-1^ Mpc^-1^ and q_0_ = 0.0). We find that most radio galaxies reside in rather poor clusters (or loosely "groups") that have three to 10 members (whose magnitudes are brighter than -19 and that lie within 0.5 Mpc of the radio galaxy). Although there is a possibility that up to 5% of all radio galaxies are field galaxies (and up to 13% are pairs), our result is consistent with all radio galaxies being in groups. The distributions in group richness and Bautz-Morgan (BM) class of radio- selected groups are different from those of optically selected groups. Radio-selected groups are preferentially of BM type I, while optically selected groups are preferentially of BM type III. The richness distributions are comparable for rich groups (with N^-19^_0.5 > 12); however, poor radio-selected groups are ~1.5 times less abundant. A second result is that the environments of FR I and FR II radio galaxies are different. FR I galaxies are found on average in richer groups than FR II galaxies. At low redshifts, FR II galaxies avoid rich groups; however, they do exist in rich groups at high redshifts. Most groups surrounding FR I galaxies (FR I groups) are of Bautz-Morgan class I, while most groups surrounding FR II galaxies (FR II groups) are of Bautz-Morgan class III. FR I groups have relatively fewer blue members than FR II groups. FR I galaxies are relatively closer to the nominal group centers than FR II galaxies. About twice as many FR II galaxies as FR I galaxies show either signatures of galaxy interactions and/or have a neighbor within 50 kpc. Together with the results presented in the first paper in this series, we argue that FR I sources are associated with centrally dominant cD-like galaxies that have been exposed to galactic cannibalism while FR II sources are more often associated with galaxies that are involved with galaxy interactions. Finally, we show that the high- and low-redshift FR I and FR II groups belong to separate subsets of groups and that each subset is different from optically selected groups. We propose that FR I groups are dynamically more evolved and FR II groups less evolved than normal groups. Since no correlations exist between the properties of group members and the radio activity of the FR I and FR II sources, radio- selected groups can still be used to study the general evolution of galaxies in groups. Since radio-selected groups are richer at high redshifts, they provide a good method of finding distant groups. Subject headings: galaxies: clusters: general-galaxies: evolution - galaxies: structure-radio continuum: galaxies
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