ar X iv : a st ro - p h / 01 04 18 3 v 1 1 0 A pr 2 00 1 On The Parent Population of Radio Galaxies and the FR I – FR II Dichotomy

We test the hypothesis that radio galaxies are a random subset of otherwise normal elliptical galaxies. Starting with the observed optical luminosity functions for elliptical galaxies, we show that the probability of an elliptical forming a radio source is a continuous, increasing function of optical luminosity, proportional to L. With this probability function and the luminosity function of normal elliptical galaxies as input to Monte Carlo simulations, we reproduce the observed distribution of radio galaxies in the radio-optical luminosity plane. Our results show that radio galaxies are a luminosity-biased but otherwise random sample of elliptical galaxies. This unified view of radio-loud and radio-quiet ellipticals also explains the well known difference of ∼ 0.5 mag in average optical luminosity between FR I and FR II radio galaxies as a simple selection effect. Specifically, FR II appear preferentially in smaller galaxies because both radio and optical luminosity functions are steep, so there is a negligible probability of observing a powerful radio source associated with a bright galaxy; no intrinsic physical differences between FR I and FR II host galaxies are required.