The Effect of Many Minor Mergers on the Size Growth of Compact Quiescent Galaxies

Abstract

Massive galaxies with a half-mass radius <~ 1kpc are observed in the early universe (z~>2), but not in the local universe. In the local universe similar-mass (within a factor of two) galaxies tend to be a factor of 4 to 5 larger. Dry minor mergers are known to drive the evolution of the size of a galaxy without much increasing the mass, but it is unclear if the growth in size is sufficient to explain the observations. We test the hypothesis that galaxies grow through dry minor mergers by simulating merging galaxies with mass ratios of q=1:1 (equal mass) to q=1:160. In our N-body simulations the total mass of the parent galaxy doubles. We confirm that major mergers do not cause a sufficient growth in size. The observation can be explained with mergers with a mass ratio of q=1:5--1:10. Smaller mass ratios cause a more dramatic growth in size, up to a factor of ~17 for mergers with a mass ratio of 1:80. For relatively massive minor mergers q ~> 1:20 the mass of the incoming child galaxies tend to settle in the halo of the parent galaxy. This is caused by the tidal stripping of the child galaxies by the time they enter the central portion of the parent. When the accretion of minor galaxies becomes more continuous, when q <~ 1:40, the foreign mass tends to concentrate more in the central region of the parent galaxy. We speculate that this is caused by dynamic interactions between the child galaxies inside the merger remnant and the longer merging times when the difference in mass is larger. These interactions cause dynamical heating which results in accretion of mass inside the galaxy core and a reduction of the parent's circular velocity and density.