Radio Galaxies: I. The Interpretation of Radio Source Data

Abstract

In preparation for the theory in the next paper (II) the extensive observational data on radio galaxies recently published are examined. The emphasis is on the quantitative information which the data provide for a theory of radio galaxies and on the qualitative features for which theory must account. Recent radio frequency spectral data are discussed first. They are shown to imply that the process which generates relativistic electrons is a very general phenomenon which gives rise to essentially similar energy distribution spectra in astrophysical systems of vastly differing parameters. Electron energy losses which modify the radio spectra are rediscussed and it is shown in particular that the effects of synchrotron and ionization losses are much more drastic than has been usually assumed, in the case where the generation time is much shorter than the lifetime of the radio source. The data rule out certain proposed evolutionary courses for radio galaxies and indicate that the ages of strong radio sources are very much less than the cosmological time scale or those suggested by the ratio of estimated energy and measured power. The variety of spectra can be satisfactorily accounted for by the effect of energy losses on initially inverse power law spectra, except for the few sources of extreme surface brightness where synchrotron self-absorption may be involved. The implications of angular diameter, brightness distribution and polarization data for theoretical models are discussed and the question of whether relativistic particles are expelled from the optical galaxy isotropically or in preferred directions is considered in the light of optical and radio data. The difference between a typical radio source in a sample complete above a given flux density and the most common source in unit volume of space is stressed using a numerical example. In the last section alleged empirical relations between radio galaxy properties are examined. Present data do not support the existence of any unique relations but do indicate the presence of certain trends.