On ths assembly history of stellar components in massive galaxies
  Matsuoka & Kawara (2010) showed that the number density of the most massive
galaxies (log M/M_sun=11.5-12.0) increases faster than that of the next massive
group (log M/M_sun=11.0-11.5) during 0 < z < 1. This appears to be in
contradiction to the apparent "downsizing effect". We attempt to understand the
two observational findings in the context of the hierarchical merger paradigm
using semi-analytic techniques. Our models closely reproduce the result of
Matsuoka & Kawara (2010). Downsizing can also be understood as larger galaxies
have, on average, smaller assembly ages but larger stellar ages. Our fiducial
models further reveal details of the history of the stellar mass growth of
massive galaxies. The most massive galaxies (log M/M_sun=11.5-12.0 at z=0),
which are mostly brightest cluster galaxies, obtain roughly 70% of their
stellar components via merger accretion. The role of merger accretion
monotonically declines with galaxy mass: 40% for log M/M_sun=11.0-11.5 and 20%
for log M/M_sun=10.5-11.0 at z=0. The specific accreted stellar mass rates via
galaxy mergers decline very slowly during the whole redshift range, while
specific star formation rates sharply decrease with time. In the case of the
most massive galaxies, merger accretion becomes the most important channel for
the stellar mass growth at z~2. On the other hand, in-situ star formation is
always the dominant channel in L* galaxies.
