Stellar subsystems of different ages in spiral and irregular galaxies

Based on archival Hubble Space Telescope images, we have performed stellar photometry for eight edge-on spiral and irregular galaxies. We have identified stars of three ages in the derived Hertzsprung-Russell diagrams and constructed their number density distribution perpendicularly to the plane of the galactic disk. The sizes of the stellar subsystems of young (up to 100 Myr), middle (0.1–1.0 Gyr), and old (up to 12 Gyr) ages have been determined. A relationship between the age of a subsystem and its size has been found in all of the galaxies studied. Our results can be explained by the model of galactic thick-disk formation through thin-disk expansion. In this case, the middle-age stellar subsystem is a transitional stage from the thin disk to the thick one.