A Study of Catalogued Nearby Galaxy Clusters in the SDSS-DR4. II.
  Cluster Substructure
  According to the current cosmological paradigm, large scale structures form
hierarchically in the Universe. Clusters of galaxies grow through a continuous
accretion of mass. Nevertheless, the rate and manner of mass accretion events
are still matters of debate. We have analysed the presence of substructures in
one of the largest sample of nearby cluster galaxies available in the
literature. We have determined the fraction of clusters with substructure and
the properties of the galaxies located in such substructures. Substructure in
the galaxy clusters was studied using the Dressler--Shectman test, which was
calibrated through extensive Monte Carlo simulations of galaxy clusters similar
to real ones. In order to avoid possible biases in the results due to differing
incompleteness among clusters, we selected two galaxy populations: a) galaxies
brighter than M$_{r} = $-20 located in clusters at $z < 0.1$ (EC1); and b)
galaxies of brightness $M_{r} < -19$ located at $z<0.07$ (EC2). In the inner
cluster regions ($r < r_{200}$) 11$\%$ and 33$\%$ of the clusters of EC1 and
EC2 respectively show substructure. This fraction is larger in the outer
cluster regions ($\approx 55\%$) for EC1 and EC2 samples. Cluster global
properties, such as $\sigma_{c}$, $f_{b}$ or $\Delta m_{12}$ do not depend on
the amount of cluster substructure. We have studied the properties of
individual galaxies located in substructures in the EC1 and EC2 galaxy
populations. The fraction of galaxies within substructures is larger in the
outer cluster regions when fainter galaxies are included. The distribution of
relative velocities of galaxies within substructures suggest that they consist
of an infalling population mixed with backsplash galaxies. We can not rule out
that the infalling galaxy population located in substructures are genuine field
ones.
