We revisit collisionless major and minor mergers of spheroidal galaxies in the context of recent observational insights on compact massive early-type galaxies at high redshift and their rapid evolution on cosmological timescales .
The simulations are performed as a series of mergers with mass-ratios of 1:1 and 1:10 for models representing pure bulges as well as bulges embedded in dark matter halos .
For major and minor mergers , respectively , we identify and analyze two different processes , violent relaxation and stripping , leading to size evolution and a change of the dark matter fraction within the observable effective radius r _ { \mathrm { e } } .
Violent relaxation - which is the dominant mixing process for major mergers but less important for minor mergers - scatters relatively more dark matter particles than bulge particles to radii r < r _ { \mathrm { e } } .
Stripping in minor mergers assembles stellar satellite particles at large radii in halo dominated regions of the massive host .
This strongly increases the size of the bulge into regions with higher dark matter fractions leaving the inner host structure almost unchanged .
A factor of two mass increase by minor mergers increases the dark matter fraction within the effective radius by 80 per cent whereas relaxation in one equal-mass merger only leads to an increase of 25 percent .
We present analytic corrections to simple one-component virial estimates for the evolution of the gravitational radii .
These estimates are shown to underpredict the evolution of the effective radii for parabolic minor mergers of bulges embedded in massive dark matter halos .
If such a two-component system grows by minor mergers alone its size growth , r _ { \mathrm { e } } \propto M ^ { \alpha } , reaches values of \alpha \approx 2.4 , significantly exceeding the simple theoretical limit of \alpha = 2 .
For major mergers the sizes grow with \alpha \lesssim 1 .
In addition , we discuss the velocity dispersion evolution and velocity anisotropy profiles .
Our results indicate that minor mergers of galaxies embedded in massive dark matter halos provide a potential mechanism for explaining the rapid size growth and the build-up of massive elliptical systems predicting significant dark matter fractions and radially biased velocity dispersions at large radii .