2018ApJ...857..102L

We investigate the properties of bright galaxies of various morphological types in Abell 1139 and Abell 2589, using pixel color-magnitude diagram (pCMD) analysis. The sample contains 32 galaxies brighter than M_r = -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 the pCMDs depend on galaxy morphology and infrared color. The mean g - r color as a function of surface brightness (µ_r) in the pCMD of a galaxy shows good 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 µ_r works better. We find that the best set of parameters for galaxy classification is a combination of the minimum color dispersion at µ_r <= 21.2 mag arcsec^–2 and the maximum color dispersion at 20.0 <= µ_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 Wide-field Infrared Survey Explorer infrared color ([4.6]-[12]). This indicates that the complexity of stellar populations in an elliptical galaxy is related to its recent star formation activities. From this observational evidence, we infer that gas-rich minor mergers or gas interactions may have usually occurred during the recent growth of massive elliptical galaxies.