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Multiple Stellar Populations of Globular Clusters from Homogenous Ca by Photometry I. M22 (NGC 6656)

We investigate the multiple stellar populations in one of peculiar globular clusters (GCs) M22 using new ground-based wide-field Ca by and HST/WFC3 photometry with equivalent passbands, confirming our previous result that M22 has a distinctive red-giant branch (RGB) split mainly due to difference in metal abundances. We also make use of radial velocity measurements of the large number of cluster membership stars by other. Our main results are followings. (i) The RGB and the subgiant branch (SGB) number ratios show that the calcium-weak (Ca-w) group is the dominant population of the cluster. However, an irreconcilable difference can be seen in the rather simple two horizontal branch (HB) classification by other. (ii) Each group has its own CN-CH anti-correlation. However, the alleged CN-CH positive correlation is likely illusive. (iii) The location of the RGB bump of the calcium-strong (Ca-s) group is significantly fainter, which may pose a challenge to the helium enhancement scenario in the Ca-s group. (iv) The positions of the center are similar. (v) The Ca-w group is slightly more centrally concentrated, while the the Ca-s is more elongated at larger radii. (vi) The mean radial velocities for both groups are similar, but the Ca-s group has a larger velocity dispersion. (vii) The Ca-s group rotates faster. The plausible scenario for the formation of M22 is that it formed via a merger of two GCs in a dwarf galaxy environment and accreted later to our Galaxy.