On the parent population of radio galaxies and the FrI-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 ellip tical forming a radio source is a continuous, increasing function of optica l luminosity, proportional to L2+/-0.4. With this probability function and the luminosity function of normal elliptical galaxies as input to Monte Car lo simulations, we reproduce the observed distribution of radio galaxies in the radio-optical luminosity plane. Our results show that radio galaxies a re a luminosity-biased but otherwise random sample of elliptical galaxies. This unified view of radio-loud and radio-quiet ellipticals also explains t he well-known difference of similar to0.5 mag in average optical luminosity between FR I and FR II radio galaxies as a simple selection effect. Specif ically, FR II galaxies appear preferentially in smaller galaxies because bo th radio and optical luminosity functions are steep, so there is a negligib le probability of observing a powerful radio source associated with a brigh t galaxy; no intrinsic physical differences between FR I and FR II host gal axies are required.