Optical identification/flux density relationship for radio galaxies

Abstract

Optical identification statistics have been summarized over a flux-density range of about 1000:1 at 408 MHz for identifications of galaxies made on the Palomar Sky Survey (PSS) prints from the 3CR, Ooty occultation, Bologna and 5C surveys. The percentage identification of galaxies seen on PSS, PI(S), decreased from about 60% for a flux density of about 15 Jy at 408 MHz to about 15% at 1 Jy and ~10% at 25 mJy. The observed PI(S) relation is compared with the predictions of theoretical models of radio luminosity function derived by Wall et al. (1977) and Robertson (1980) based on radio source counts and luminosity distribution of strong radio sources. Rather than assuming a standard optical luminosity for all radio galaxies, we have taken into account the available information on their bivariate radio-optical luminosity function. It is found that the observed PI(S) data do not support model 5 of wall et., al. in spite of its consistency with radio source counts, implying that there occures no evolution of radio sources for luminosities < 10²⁵ W Hz^-1 sr^-1 at 408 MHz. The observation of PI(S) are in resonable aggrement with model 4 (b) of wall et., al. and with the free-form model of Robertson but indicate that the local luminosity function at 408 MHz in the range of about 10^23-25 W Hz^-1 sr^-1 is lower by a factor of about two than that assumed by them.