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