2MASS photometry and age estimate of globular clusters in the outer halo
  of M31
  We present the first photometric results in J, H, and K_s from 2MASS imaging
of 10 classical globular clusters in the far outer regions of M31. Combined
with the V and I photometric data from previous literature, we constructed the
color-color diagram between J-K_s and V-I. By comparing the integrated
photometric measurements with evolutionary models, we estimate the ages of
these clusters. The results showed that, all of these clusters are older than
$3\times 10^9$ yrs, of which 4 are older than 10 Gyrs and the other 6 are in
intermediate ages between 3-8 Gyrs. The masses for these outer halo GCs are
from $7.0\times 10^4 M_sun$ to $1.02\times 10^6 M_sun$. We argued that, GC2 and
GC3, the ages, metallicities and the distance moduli of which are nearly the
same, were accreted from the same satellite galaxy, if they did not form {\it
in situ}. The statistical results show that, ages and metallicities for these
10 M31 outer halo GCs do not vary with projected radial position, and the
relationship between age and metallicity doest not exit.
