We use direct N-body simulations to investigate the evolution of star
clusters with large size-scales with the particular goal of understanding the
so-called extended clusters observed in various Local Group galaxies, including
M31 and NGC6822. The N-body models incorporate a stellar mass function, stellar
evolution and the tidal field of a host galaxy. We find that extended clusters
can arise naturally within a weak tidal field provided that the tidal radius is
filled at the start of the evolution. Differences in the initial tidal
filling-factor can produce marked differences in the subsequent evolution of
clusters and the size-scales that would be observed. These differences are more
marked than any produced by internal evolution processes linked to the
properties of cluster binary stars or the action of an intermediate-mass black
hole, based on models performed in this work and previous work to date. Models
evolved in a stronger tidal field show that extended clusters cannot form and
evolve within the inner regions of a galaxy such as M31. Instead our results
support the suggestion many extended clusters found in large galaxies were
accreted as members of dwarf galaxies that were subsequently disrupted. Our
results also enhance the recent suggestion that star clusters evolve to a
common sequence in terms of their size and mass.Comment: 12 pages, 8 figures, accepted by MNRA
