Cosmological origin of the stellar velocity dispersions in massive early-type galaxies

We show that the observed upper bound on the line-of-sight velocity dispersion of the stars in an early-type galaxy, sigma(e) less than or similar to 400 km s(-1), may have a simple dynamical origin within the LambdaCDM cosmological model, under two main hypotheses. The first is that most of the stars now in the luminous parts of a giant elliptical galaxy formed at redshift z greater than or similar to 6. Subsequently, the stars behaved dynamically just as an additional component of the dark matter. The second hypothesis is that the mass distribution characteristic of a newly formed dark matter halo forgets such details of the initial conditions as the stellar "collisionless matter'' that was added to the dense parts of earlier generations of halos. We also assume that the stellar velocity dispersion does not evolve much at z less than or similar to 6, because a massive host halo grows mainly by the addition of material at large radii well away from the stellar core of the galaxy. These assumptions lead to a predicted number density of elliptical galaxies as a function of stellar velocity dispersion that is in promising agreement with the Sloan Digital Sky Survey data.