Stellar Structure of Irregular Galaxies. I. Face-on Galaxies

Abstract

Stellar photometry of nearby irregular galaxies of the Local Group is used to identify and study the young and old stellar populations of these galaxies. An analysis of the spatial distributions of stars of different ages in face-on galaxies shows that the young stellar populations in irregular galaxies are concentrated toward the center, and form local inhomogeneities in star-forming regions, while the old stellar populations--red giants--form extended structures around the irregular galaxies. The sizes of these structures exceed the visible sizes of the galaxies at the 25m/arcsec^2 isophote by a factor of two to three. The surface density of the red giants decreases exponentially from the center toward the edge, similar to the disk components in spiral galaxies.