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\nuniverse (z~>2), but not in the local universe. In the local universe\nsimilar-mass (within a factor of two) galaxies tend to be a factor of 4 to 5\nlarger. Dry minor mergers are known to drive the evolution of the size of a\ngalaxy without much increasing the mass, but it is unclear if the growth in\nsize is sufficient to explain the observations. We test the hypothesis that\ngalaxies grow through dry minor mergers by simulating merging galaxies with\nmass ratios of q=1:1 (equal mass) to q=1:160. In our N-body simulations the\ntotal mass of the parent galaxy doubles. We confirm that major mergers do not\ncause a sufficient growth in size. The observation can be explained with\nmergers with a mass ratio of q=1:5--1:10. Smaller mass ratios cause a more\ndramatic growth in size, up to a factor of ~17 for mergers with a mass ratio of\n1:80. For relatively massive minor mergers q ~> 1:20 the mass of the incoming\nchild galaxies tend to settle in the halo of the parent galaxy. This is caused\nby the tidal stripping of the child galaxies by the time they enter the central\nportion of the parent. When the accretion of minor galaxies becomes more\ncontinuous, when q <~ 1:40, the foreign mass tends to concentrate more in the\ncentral region of the parent galaxy. We speculate that this is caused by\ndynamic interactions between the child galaxies inside the merger remnant and\nthe longer merging times when the difference in mass is larger. These\ninteractions cause dynamical heating which results in accretion of mass inside\nthe galaxy core and a reduction of the parent's circular velocity and density.\n