What is a Galaxy? Cast your vote here...
  Although originally classified as galaxies, Ultra Compact Dwarfs (UCDs) share
many properties in common with globular star clusters. The debate on the origin
and nature of UCDs, and the recently discovered ultra-faint dwarf spheroidal
(dSph) galaxies which contain very few stars, has motivated us to ask the
question `what is a galaxy?' Our aim here is to promote further discussion of
how to define a galaxy and, in particular, what separates it from a star
cluster. Like most previous definitions, we adopt the requirement of a
gravitationally bound stellar system as a minimum. In order to separate a dwarf
galaxy from a globular cluster, we discuss other possible requirements, such as
a minimum size, a long two-body relaxation time, a satellite system, the
presence of complex stellar populations and non-baryonic dark matter. We
briefly mention the implications of each of these definitions if they are
adopted. Some special cases of objects with an ambiguous nature are also
discussed. Finally, we give our favoured criteria, and in the spirit of a
`collective wisdom', invite readers to vote on their preferred definition of a
galaxy via a dedicated website.
