The structure of star clusters in the outer halo of M31
  We present a structural analysis of halo star clusters in M31 based on deep
Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging. The
clusters in our sample span a range in galactocentric projected distance from
13 to 100 kpc and thus reside in rather remote environments. Ten of the
clusters are classical globulars, while four are from the Huxor et al. (2005,
2008) population of extended, old clusters. For most clusters, contamination by
M31 halo stars is slight, and so the profiles can be mapped reliably to large
radial distances from their centres. We find that the extended clusters are
well fit by analytic King (1962) profiles with ~20 parsec core radii and ~100
parsec photometric tidal radii, or by Sersic profiles of index ~1 (i.e.
approximately exponential). Most of the classical globulars also have large
photometric tidal radii in the range 50-100 parsec, however the King profile is
a less good fit in some cases, particularly at small radii. We find 60% of the
classical globular clusters exhibit cuspy cores which are reasonably well
described by Sersic profiles of index ~2-6. Our analysis also reinforces the
finding that luminous classical globulars, with half-light radii <10 parsec,
are present out to radii of at least 100 kpc in M31, which is in contrast to
the situation in the Milky Way where such clusters (other than the unusual
object NGC 2419) are absent beyond 40 kpc.
