E+S galaxy pairs: are they the precursors of fossil groups?
  Galaxy pairs may represent a way station in the evolutionary path from poor
groups to giant isolated ellipticals (or fossil groups). To test this
evolutionary scenario, we investigated the environment of 4 galaxy pairs
composed of a giant elliptical galaxy and its spiral companion. The pairs are
very similar from the optical and dynamical point of view, but have very
different X-ray properties. The faint galaxy population around the pairs was
observed with VIMOS on the VLT. These observations show that the presence of
extended diffuse X-ray emission from an IGM is not necessarily connected to the
presence of a numerous faint galaxy population. The study of luminosity
functions (LFs) indicate that our X-ray luminous pairs are more dynamically
evolved than a sample of poor groups with comparable X-ray luminosities from
the literature. However, our X-ray faint pairs resemble the LF of those X-ray
bright groups and may represent a phase in the dynamical evolution of these
groups, where the recent or ongoing interaction, in which the pair E is
involved, has destroyed or at least decreased the luminosity of the IGM. The
X-ray faint groups' LF is also consitent with their evolution into a fossil
group.
