Fossil Group Origins VII. Galaxy substructures in fossil systems
  Fossil groups are expected to be the final product of galaxy merging within
galaxy groups. In simulations, they are predicted to assemble their mass at
high redshift. This early formation allows for the innermost $M^\ast$ galaxies
to merge into a massive central galaxy. Then, they are expected to maintain
their fossil status because of the few interactions with the large-scale
structure. In this context, the magnitude gap between the two brightest
galaxies of the system is considered a good indicator of its dynamical status.
As a consequence, the systems with the largest gaps should be dynamically
relaxed. In order to examine the dynamical status of these systems, we
systematically analyze, for the first time, the presence of galaxy
substructures in a sample of 12 spectroscopically-confirmed fossil systems with
redshift $z \le 0.25$. We apply a number of tests in order to investigate the
substructure in fossil systems in the two-dimensional space of projected
positions out to $R_{200}$. Moreover, for a subsample of 5 systems with at
least 30 spectroscopically-confirmed members we also analyze the substructure
in the velocity and in the three-dimensional velocity-position spaces.
Additionally, we look for signs of recent mergers in the regions around the
central galaxies. We find that an important fraction of fossil systems show
substructure. The fraction depends critically on the adopted test, since each
test is more sensible to a particular type of substructure. Our interpretation
of the results is that fossil systems are not, in general, as relaxed as
expected from simulations. Our sample of 12 spectroscopically-confirmed fossil
systems need to be extended in order to compute an accurate fraction, but our
conclusion is that it is similar to the fraction of substructure detected in
non-fossil clusters. THIS ABSTRACT IS TRUNCATED.
