Studies of the stellar populations of galaxies using two-color diagrams

Variations in the photometric parameters of stellar systems as a function of their evolution and the stellar populations comprising them are investigated. A set of seven evolutionary models with an exponential decrease in the star-formation rate and 672 models with a secondary burst of star formation are considered. The occurrence of a secondary burst of star formation can shift the position of a stellar system on two-color diagrams to the right or left of the normal color sequence for galaxies and the absorption line. This makes it possible to estimate the composition of the stellar population of a galaxy with a nonmonotonic star-formation history from its position on two-color diagrams. Surface photometry in both the optical (UBV RI) and near-IR (JHK) is used to study the stellar populations and star-formation histories in the structural components (nucleus, bulge, disk, spiral arms, bar, ring) of 26 galaxies of various morphological types (from S0 to Sd). Components (nucleus, bulge, bar) with color characteristics corresponding to stellar systems with secondary bursts of star formation are indicated in 10 of the 26 galaxies. The parameters of these secondary bursts are estimated. Five of the 10 galaxies with complex star-formation histories display clear structural perturbations. Appreciable differences in the photometric characteristics of relatively red early-type galaxies (S0—Sb) and relatively blue later-type galaxies (Sb—Sd) have been found. Galaxies of both early and late types are encountered among the Sb galaxies. Lenticular galaxies do not display different photometric characteristics from early-type spiral galaxies.