  The Cheshire Cat is a relatively poor group of galaxies dominated by two
luminous elliptical galaxies surrounded by at least four arcs from
gravitationally lensed background galaxies that give the system a humorous
appearance. Our combined optical/X-ray study of this system reveals that it is
experiencing a line of sight merger between two groups with a roughly equal
mass ratio with a relative velocity of ~1350 km/s. One group was most likely a
low-mass fossil group, while the other group would have almost fit the
classical definition of a fossil group. The collision manifests itself in a
bimodal galaxy velocity distribution, an elevated central X-ray temperature and
luminosity indicative of a shock, and gravitational arc centers that do not
coincide with either large elliptical galaxy. One of the luminous elliptical
galaxies has a double nucleus embedded off-center in the stellar halo. The
luminous ellipticals should merge in less than a Gyr, after which observers
will see a massive 1.2-1.5 x 10^14 solar mass fossil group with an M_r = -24.0
brightest group galaxy at its center. Thus, the Cheshire Cat offers us the
first opportunity to study a fossil group progenitor. We discuss the
limitations of the classical definition of a fossil group in terms of magnitude
gaps between the member galaxies. We also suggest that if the merging of fossil
(or near-fossil) groups is a common avenue for creating present-day fossil
groups, the time lag between the final galactic merging of the system and the
onset of cooling in the shock-heated core could account for the observed lack
of well-developed cool cores in some fossil groups.

Astrophysics of Galaxies;Cosmology and Nongalactic Astrophysics;High Energy Astrophysical Phenomena