2MASS photometry and age estimation of the globular clusters in the outer halo of M31

We present the first photometric results in J, H and K_s from 2MASS (Two Micron All Sky Survey) imaging of 10 classical globular clusters (GCs) in the far outer regions of M31. Combined with the V and I photometric data from previous literature, we constructed a 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 × 10⁹ yr, and among them four are older than 10 Gyr and the other six have intermediate ages between 3–8 Gyr. The masses for these outer-halo GCs are from 7.0 × 10⁴ M_⊙ to 1.02 × 10⁶ M_⊙. We argued that GC2 and GC3, whose ages, metallicities and distance moduli are almost the same, were accreted from the same satellite galaxy, if they did not form in situ. The statistical results show that the ages and metallicities of these 10 M31 outer-halo GCs do not vary with projected radial position, and a relationship between age and metallicity does not exist.