Dry minor mergers and size evolution of high-z compact massive
  early-type galaxies
  Recent observations show evidence that high-z (z\sim 2 - 3) early-type
galaxies (ETGs) are more compact than those with comparable mass at z\sim 0.
Such a size evolution is most likely explained by the `Dry Merger Sceanario'.
However, previous studies based on this scenario are not able to consistantly
explain both the properties of the high-z compact massive ETGs and the local
ETGs. We investigate the effect of multiple sequential dry minor mergers on the
size evolution of the compact massive ETGs. From an analysis of the Millennium
Simulation Database, we show that such minor (stellar mass ratio $M_{2}/M_{1} <
1/4$) mergers are extremely common during hierarchical structure formation. We
perform N-body simulations of sequential minor mergers with parabolic and
head-on orbits, including a dark matter component and a stellar component.
Typical mass ratios of the minor mergers are $1/20 < M_{2}/M_{1} < 1/10$. We
show that sequential minor mergers of compact satellite galaxies are the most
efficient at promoting size growth and decreasing the velocity dispersion of
the compact massive ETGs in our simulations. The change of stellar size and
density of the merger remnants is consistent with recent observations.
Furthermore, we construct the merger histories of candidates for the high-z
compact massive ETGs using the Millennium Simulation Database, and estimate the
size growth of the galaxies by the dry minor merger scenario. We can reproduce
the mean size growth factor between $z=2$ and $z=0$, assuming the most
efficient size growth obtained during sequential minor mergers in our
simulations. However, we note that our numerical result is only valid for
merger histories with typical mass ratios between 1/20 and 1/10 with parabolic
and head-on orbits, and that our most efficient size growth efficiency is
likely to an upper limit.
