The environments of radio galaxies

Abstract

An investigation into the environments of a large number of radio galaxies is described. These are drawn from radio complete samples, with no initial knowledge of source environment required. Instead, an objective method is used to parameterise this quantity, which is then compared with the sources' radio and optical properties.

The main source of objects is an all-sky sample, defined at high frequency, which provides an opportunity to investigate some less well studied members of the radio-source population. As a preliminary study, U.K. Schmidt plates are used to look for optical counterparts for 34 sample members, which have no reliable identifications. VLA observations are made of 71 sources, comprising all objects between +10°> 5 > -46° which are not known to be compact. These data allow unambiguous morphological classifications to be made for the lower-redshift sources for which clustering analyses are performed.

The environment parameter used is the angular cross-correlation amplitude, corrected for the effects of redshift using the galaxy luminosity function. Results for radio sources correlated with the Lick counts are presented; for a number of objects deeper U.K. Schmidt material is also considered. Particular attention is paid to the origin and magnitude of possible sources of error in the measurement of cluster strength.

Compact sources are found not to appear in regions of enhanced galaxy density. This is stong evidence against 'unified' models for these objects. For extended sources, there is a correlation between the local galaxy density and the radio structure. FRI galaxies appear in regions of generally enhanced galaxy density. FRII sources are typically found in environments similar to those of "ordinary" members of the galaxy population, although some FRII sources do appear in richer environments. In terms of local environment there is no definite distinction between the classical and non-classical double sources.

Surface photometry of 29 sample members is presented and discussed. Both the total magnitude and the optical structure of a source are found to be closely related to its local environment. FRI galaxies, in rich environments, are bright and have large characteristic sizes, while FRII galaxies are fainter and less extended. A number of good examples of mergers are found, and in each case the associated radio structure is of type FRI. Correlations with optical spectral type are also presented.

The implications of these results for the morphology of radio galaxies is discussed.