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

TL;DR

This study uses N-body simulations to investigate how numerous minor mergers contribute to the size growth of compact quiescent galaxies, finding that small mass ratio mergers significantly increase galaxy size and central mass concentration.

Contribution

The paper demonstrates through simulations that many minor mergers with small mass ratios can explain the observed size evolution of compact galaxies from early to late universe.

Findings

Minor mergers with mass ratios of 1:5 to 1:10 cause sufficient size growth.

Mergers with ratios smaller than 1:20 lead to significant size increase, up to 17 times.

Mass from minor mergers tends to settle in the halo or central region depending on the merger ratio.

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.