  We reveal the role of clustering of sub-clumps, which is expected in the cold
dark matter (CDM) universe, in forming a bright elliptical galaxy (BEG) from a
low-spin seed galaxy.
  This can be done by comparing the evolution of a low-spin seed galaxy
including small-scale density fluctuations expected in the CDM universe
  (Model 1) with that of a completely uniform one (Model 2), using numerical
experiments.
  We show that Model 2 cannot reproduce the properties of BEGs and forms a disk
which is too compact and too bright due to the conservation of the
initial-small angular momentum.
  In Model 1 clustering of the sub-clumps caused by initial small-scale density
fluctuations leads to angular momentum transfer from the baryon component to
the dark matter and consequently a nearly spherical system supported by random
motions is formed.
  Moreover the collisionless property of the stars formed in the sub-clumps
prevents the dissipative contraction of the system, leading to a large measured
half-light radius.
  As a result, the end-product is quite well reproduces the observed properties
of BEGs, such as the de Vaucouleurs light-profile, typical color and
metallicity gradients, the large half-light radius, the small ratio of the
rotational velocity to the velocity dispersion (V/\sigma). We conclude that the
clustering of sub-clumps, i.e., the hierarchical clustering, plays a crucial
role in the formation of BEGs from a low-spin seed galaxy.

Astrophysics