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5.3.5 Environments

For a unification scheme to be correct, the environments of the unified classes - here, FR IIs and radio quasars - have to match. (10) The observational situation here is unclear. In an oft-quoted study, Prestage and Peacock (1988) found that compact/flat-spectrum radio sources appear to lie in regions of galaxy density a factor of ~ 2 lower than those typical of FR Is and possibly even lower than for FR II sources. However, this was based on small samples of only ~ 10 compact flat-spectrum sources, which included BL Lacs, local FSRQ, and even some radio galaxies. The Prestage and Peacock (1988) data on FSRQ alone are too limited to derive a spatial correlation amplitude for comparison to the FR IIs.

Other studies at low redshift showed either no significant difference in environment (Smith and Heckman 1990), or only a marginal difference (Yates et al. 1989) that disappears when the radio galaxies without definite FR II structure are excluded. At higher redshift (0.35 ltapprox z ltapprox 0.5), the environments of powerful radio galaxies are as rich as those observed around radio-loud quasars in the same redshift interval (Yates et al. 1989).

Thus there is no evidence for concluding that the environments of FR IIs and radio quasars differ. Obviously, additional data for larger samples covering a range in redshift are needed to confirm that the environments are similar.

10 Even within the unified scheme, environment may have an effect on the relative numbers of radio galaxies and blazars. For example, the opening angle of the torus might depend on gas density external to the active nucleus. In that case, the number ratio of blazars to radio galaxies could differ between cluster and field. If the opening angle were smaller in clusters due to extra quenching, for example, radio galaxies would be found more often in clusters than are blazars. This is a matter of degree, however; the fact that both radio galaxies and blazars should be found in clusters and in the field would not change.