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We investigate the multiple stellar populations in one of peculiar globularclusters (GCs) M22 using new ground-based wide-field Ca by and HST/WFC3photometry with equivalent passbands, confirming our previous result that M22has a distinctive red-giant branch (RGB) split mainly due to difference inmetal abundances. We also make use of radial velocity measurements of the largenumber of cluster membership stars by other. Our main results are followings.(i) The RGB and the subgiant branch (SGB) number ratios show that thecalcium-weak (Ca-w) group is the dominant population of the cluster. However,an irreconcilable difference can be seen in the rather simple two horizontalbranch (HB) classification by other. (ii) Each group has its own CN-CHanti-correlation. However, the alleged CN-CH positive correlation is likelyillusive. (iii) The location of the RGB bump of the calcium-strong (Ca-s) groupis significantly fainter, which may pose a challenge to the helium enhancementscenario 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 ismore elongated at larger radii. (vi) The mean radial velocities for both groupsare similar, but the Ca-s group has a larger velocity dispersion. (vii) TheCa-s group rotates faster. The plausible scenario for the formation of M22 isthat it formed via a merger of two GCs in a dwarf galaxy environment andaccreted later to our Galaxy.

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