The inefficiency of satellite accretion in forming extended star
  clusters
  The distinction between globular clusters and dwarf galaxies has been
progressively blurred by the recent discoveries of several extended star
clusters, with size (20-30 pc) and luminosity (-6 < Mv < -2) comparable to the
one of faint dwarf spheroidals. In order to explain their sparse structure, it
has been suggested that they formed as star clusters in dwarf galaxy satellites
that later accreted onto the Milky Way. If these clusters form in the centre of
dwarf galaxies, they evolve in a tidally-compressive environment where the
contribution of the tides to the virial balance can become significant, and
lead to a super-virial state and subsequent expansion of the cluster, once
removed. Using N-body simulations, we show that a cluster formed in such an
extreme environment undergoes a sizable expansion, during the drastic variation
of the external tidal field due to the accretion process. However, we show that
the expansion due to the removal of the compressive tides is not enough to
explain the observed extended structure, since the stellar systems resulting
from this process are always more compact than the corresponding clusters that
expand in isolation due to two-body relaxation. We conclude that an accreted
origin of extended globular clusters is unlikely to explain their large spatial
extent, and rather favor the hypothesis that such clusters are already extended
at the stage of their formation.
