  The differences between cold (CDM) and warm (WDM) dark matter in the
formation of a group of galaxies is examined by running two identical
simulations where in the WDM case the initial power spectrum has been altered
to mimic a 1keV dark matter particle. The CDM initial conditions were
constrained to reproduce at z = 0 the correct local environment within which a
"Local Group" (LG) of galaxies may form. Two significant differences between
the two simulations are found. While in the CDM case a group of galaxies that
resembles the real LG forms, the WDM run fails to reproduce a viable LG,
instead forming a diffuse group which is still expanding at z = 0. This is
surprising since, due to the suppression of small scale power in its power
spectrum, WDM is naively expected to only affect the collapse of small haloes
and not necessarily the dynamics on a scale of a group of galaxies. Furthermore
the concentration of baryons in halo center's is greater in CDM than in WDM and
the properties of the disks differ.
