  The Local Group is a unique environment in which to study the astrophysics of
galaxy formation. The proximity of the Milky Way and M31 causes a large
fraction of the low-mass halo population to interact with more massive dark
matter haloes, which increases their concentrations and strips them of gas and
other material. Some low-mass haloes pass through the haloes of the Milky Way
or M31 and are either ejected into the field or exchanged between the two
primary hosts. We use high resolution gas-dynamical simulations to describe a
new class of field halo that passed through the haloes of both the Milky Way
and M31 at early times and is almost twice as concentrated as isolated field
haloes. These 'Hermeian' haloes are distributed anisotropically at greater
distances from the Local Group barycentre than the primary haloes and appear to
cluster close to the Milky Way and M31 in projection. We show that some
Hermeian haloes can host galaxies that are promising targets for indirect dark
matter searches and are competitive with signals from other dwarf galaxies.
Hermeian galaxies in the Local Group should be detectable by forthcoming
wide-field imaging surveys.
