Massive galaxies with a half-mass radius { \raise - 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } } 1 kpc are observed in the early universe ( z { \raise - 2.15 pt \hbox { $ \buildrel > \over { \sim } $ } } 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 \sim 17 for mergers with a mass ratio of 1:80 .
For relatively massive minor mergers q { \raise - 2.15 pt \hbox { $ \buildrel > \over { \sim } $ } } 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 { \raise - 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } } 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 .