Fossil Groups in the Millennium Simulation: Their environment and its
  evolution
  Fossil systems are defined to be X-ray bright galaxy groups with a
2-magnitude difference between their two brightest galaxies within half the
projected virial radius,and represent an interesting extreme of the population
of galaxy agglomerations.However,the physical conditions and processes leading
to their formation are still poorly constrained.We compare the outskirts of
fossil systems with that of normal groups to understand whether environmental
conditions play a significant role in their formation.We study galaxy groups in
both,numerical simulations and observations.We use a variety of statistical
tools including the spatial cross-correlation function and the local density
parameter \Delta_5 to probe differences in the density and structure of the
environments of normal and fossil systems in the Millennium simulation.We find
that the number density of galaxies surrounding fossil systems evolves from
greater than that observed around normal systems at z=0.69, to lower than the
normal systems by z=0.Both fossil and normal systems exhibit an increment in
their otherwise radially declining local density measure (\Delta_5) at
distances of order 2.5r_{vir} from the system centre.We show that this
increment is more noticeable for fossil systems than normal systems and
demonstrate that this difference is linked to the earlier formation epoch of
fossil groups.Despite the importance of the assembly time, we show that the
environment is different for fossil and non-fossil systems with similar masses
and formation times along their evolution.We also confirm that the physical
characteristics identified in the Millennium simulation can also be detected in
SDSS observations.Our results confirm the commonly held belief that fossil
systems assembled earlier than normal systems but also show that the
surroundings of fossil groups could be responsible for the formation of their
large magnitude gap.
