On The Parent Population of Radio Galaxies and the FRI - FRII Dichotomy

Riccardo Scarpa; C. Megan Urry

doi:10.1086/321606

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arXiv · astro-ph/0104183

On The Parent Population of Radio Galaxies and the FRI - FRII Dichotomy

Riccardo Scarpa·C. Megan Urry

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Abstract

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 squared. 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 FRI and FRII radio galaxies as a simple selection effect. Specifically, FRII 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 FRI and FRII host galaxies are required.

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Riccardo Scarpa, C. Megan Urry. 2001-04-10. On The Parent Population of Radio Galaxies and the FRI - FRII Dichotomy. https://doi.org/10.1086/321606

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