Color Dispersion as an Indicator of Stellar Population Complexity: Insights from the Pixel Color-Magnitude Diagrams of 32 Bright Galaxies in Abell 1139 and Abell 2589

We investigate the properties of bright galaxies with various morphological types in Abell 1139 and Abell 2589, using the pixel color-magnitude diagram (pCMD) analysis technique. The sample is 32 galaxies brighter than Mr = -21.3 mag with spectroscopic redshifts, which are deeply imaged in the g and r bands using the MegaCam mounted on the Canada-France-Hawaii Telescope. After masking contaminants with two-step procedures, we examine how the detailed properties in pCMDs depend on galaxy morphology and infrared color. The mean g - r color as a function of surface brightness (mu_r) in the pCMD of a galaxy shows fine performance in distinguishing between early- and late-type galaxies, but it is not perfect because of the similarity between elliptical galaxies and bulge-dominated spiral galaxies. On the other hand, the g - r color dispersion as a function of mu_r works better. We find that the best set of parameters for galaxy classification is the combination of the minimum color dispersion at mu_r <= 21.2 mag/arcsec^2 and the maximum color dispersion at 20.0 <= mu_r <= 21.0 mag/arcsec^2: the latter reflects the complexity of stellar populations at the disk component in a typical spiral galaxy. Finally, the color dispersion measurements of an elliptical galaxy appear to be correlated with the WISE infrared color ([4.6] - [12]). This indicates that the complexity of stellar populations in an elliptical galaxy is related with its recent star formation activities. From this observational evidence, we infer that gas-rich minor mergers or gas interactions may have usually happened during the recent growth of massive elliptical galaxies.