The Mass Assembly History of Spheroidal Galaxies: Did Newly Formed Systems Arise via Major Mergers?

Abstract

We examine the properties of a morphologically selected sample of 0.4 < z < 1.0 spheroidal galaxies in the GOODS fields in order to ascertain whether or not their increase in abundance with time arises primarily from major mergers. To address this question, we determine the scaling relations between the dynamical mass M_(dyn), determined from stellar velocity dispersions, and the stellar mass M_*, determined from optical and infrared photometry. We exploit these relations across the larger sample for which we have stellar masses in order to construct the first statistically robust estimate of the evolving dynamical mass function (MF) over 0 < z < 1. The trends observed match those seen in the stellar mass functions of Bundy et al. regarding the top-down growth in the abundance of spheroidal galaxies. By referencing our dynamical masses to the halo virial mass, M_(vir), we compare the growth rate in the abundance of spheroidals to that predicted by the assembly of dark matter halos. Our comparisons demonstrate that major mergers do not fully account for the appearance of new spheroidals since z ~ 1 and that additional mechanisms, such as morphological transformations, are required to drive the observed evolution.