The Optical Luminosity Function of Virialized Systems

We determine the optical luminosity function of virialized systems over the full range of density enhancements, from single galaxies to clusters of galaxies. The analysis is based on galaxy systems identified from the Nearby Optical Galaxy (NOG) sample, which is the largest, all-sky catalog of objectively- identified bound objects presently available. We find that the B-band luminosity function of systems is insensitive to the choice of the group-finding algorithms and is well described, over the absolute-magnitude range -24.5 < M < -18.5, by a Schechter function with alpha=-1.4, Mstar=-23.1 and phistar=4.8 10^{-4} Mpc^-3 The characteristic luminosity of virialized systems is ~3 times brighter than that of the luminosity function of galaxies. Our results show that half of the luminosity of the universe is generated in systems with L<2.9 Lstar and that 10% of the overall luminosity density is supplied by systems with L>30 Lstar. We find a significant environmental dependence in the luminosity function of systems, in the sense that overdense regions, as measured on scales of 5 Mpc, preferentially host brighter, and presumably more massive, virialized systems.