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

TL;DR

This study investigates the evolution of spheroidal galaxies between redshifts 0.4 and 1.0, finding that major mergers alone cannot explain their increased abundance, implying other processes like morphological transformations are involved.

Contribution

It provides the first robust estimate of the evolving dynamical mass function of spheroidal galaxies and compares their growth to dark matter halo assembly.

Findings

Major mergers do not fully account for spheroidal galaxy growth.

Additional mechanisms like morphological transformations are necessary.

The dynamical mass function evolution matches stellar mass function trends.

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 their increase in abundance with time arises primarily from mergers. To address this question we determine scaling relations between the dynamical mass determined from stellar velocity dispersions, and the stellar mass 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 over 0<z<1. The trends observed match those seen in the stellar mass functions of Bundy et al. 2005 regarding the top-down growth in the abundance of spheroidal galaxies. By referencing our dynamical masses to the halo virial mass 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.