Discovery of the most isolated globular cluster in the local universe
  We report the discovery of two new globular clusters in the remote halos of
M81 and M82 in the M81 Group based on Hubble Space Telescope archive images.
They are brighter than typical globular clusters (MV = -9.34 mag for GC-1 and
M_V = -10.51 mag for GC-2), and much larger than known globular clusters with
similar luminosity in the MilkyWay Galaxy and M81. Radial surface brightness
profiles for GC-1 and GC-2 do not show any features of tidal truncation in the
outer part. They are located much farther from both M81 and M82 in the sky,
compared with previously known star clusters in these galaxies. Color-magnitude
diagrams of resolved stars in each cluster show a well-defined red giant branch
(RGB), indicating that they are metal-poor and old. We derive a low metallicity
with [Fe/H] $\simeq -2.3$ and an old age ~14 Gyr for GC-2 from the analysis of
the absorption lines in its spectrum in the Sloan Digital Sky Survey in
comparison with the simple stellar population models. The I-band magnitude of
the tip of the RGB for GC-2 is 0.26 mag fainter than that for the halo stars in
the same field, showing that GC-2 is ~400 kpc behind the M81 halo along our
line of sight. The deprojected distance to GC-2 from M81 is much larger than
any other known globular clusters in the local universe. This shows that GC-2
is the most isolated globular cluster in the local universe.
