Exploring Origins for the Hubble Sequence

Abstract

An analytic formalism which allows calculation of separate mass functions for disk, spheroid and dwarf galaxies is applied to the cold dark matter model. Dwarfs are assumed to form within halos with virial temperatures below a critical value T _c ≃ 10^5.4K. Two sets of criteria are used to differentiate spheroids from disks — one based on cooling timescales and another based on initial perturbation height. The cooling timescale model fails to reproduce simple observed properties of the galaxy distribution. The model based on perturbation height is relatively successful and predicts a “natural” morphological bias which reproduces the observed morphology-density effect in clusters. This model suggests that the transition from the disk-forming to spheroid-forming regimes occurs at a critical pressure which is independent of environment.