Is There a Universal Mass Function?

Abstract

Following an old idea of Fritz Zwicky, we make an attempt to establish a universal mass function for astronomical objects on all scales. The object classes considered are: solar system planets and small bodies, exoplanets, brown dwarfs, stars and stellar remnants, open and globular clusters, molecular clouds, galaxies, groups and clusters of galaxies. For comparison we also include CDM halos taken from numerical simulations. We show that the mass functions of individual object classes, when properly normalized, can indeed be concatenated to build a surprisingly continuous mass function of the universe, from approximately M = 10^(-20)Msun (sub-kilometer size asteroids) up to M = 10^(16)Msun (rich clusters of galaxies), covering 36 orders of magnitude in mass. Most individual mass functions roughly follow a power law of the form phi(M) propto M^(-2). A notable exception are planets and small bodies which seem to obey a flatter distribution. CDM halos from high-resolution numerical simulations show a very similar relation, again of universal slope -2, from clusters of galaxies all the way down to the planetary mass scale. On the scale of stars and star clusters this is a remarkable coincidence, as the formation processes involved are thought to be totally different (bottom-up gravitational clustering of DM halos versus top-down gravoturbulent fragmentation of gas clouds).