GC-1 and GC-2 are two globular clusters ( GCs ) in the remote halo of M81 and M82 in the M81 group discovered by Jang et al .
using the Hubble Space Telescope ( HST ) images .
These two GCs were observed as part of the Beijing–Arizona–Taiwan–Connecticut ( BATC ) Multicolor Sky Survey , using 14 intermediate-band filters covering a wavelength range of 4000–10000 Å .
We accurately determine these two clusters ’ ages and masses by comparing their spectral energy distributions ( from 2267 to 20000 Å , comprising photometric data in the near-ultraviolet of the Galaxy Evolution Explorer , 14 BATC intermediate-band , and Two Micron All Sky Survey near-infrared JHK _ { s } filters ) with theoretical stellar population-synthesis models , resulting in ages of 15.50 \pm 3.20 for GC-1 and 15.10 \pm 2.70 Gyr for GC-2 .
The masses of GC-1 and GC-2 obtained here are 1.77 - 2.04 \times 10 ^ { 6 } and 5.20 - 7.11 \times 10 ^ { 6 } ~ { } M _ { \odot } , respectively .
In addition , the deep observations with the Advanced Camera for Surveys and Wide Field Camera 3 on the HST are used to provide the surface brightness profiles of GC-1 and GC-2 .
The structural and dynamical parameters are derived from fitting the profiles to three different models ; in particular , the internal velocity dispersions of GC-1 and GC-2 are derived , which can be compared with ones obtained based on spectral observations in the future .
For the first time , in this paper , the r _ { h } versus M _ { V } diagram shows that GC-2 is an ultra-compact dwarf in the M81 group .