2003AJ....125.1247H

The morphological properties and spatial distribution of stellar populations are studied for DDO 165 and DDO 181, two dwarf irregular (dIrr) galaxies outside the Local Group. The study uses B and R photometry from which are derived (1) color-magnitude diagrams of the resolved stars, (2) surface brightness profiles for the unresolved stars, and (3) surface brightness fluctuations (SBFs) originating in the unresolved population. This information allows us to reach the following conclusions for both galaxies: (1) the distances are estimated to be 4.8±0.8 Mpc for DDO 165 and 4.4±0.4 Mpc for DDO 181, which imply that the first is a field galaxy lying between the M81 and M101 groups and the second is a member of the Canes Venatici group; (2) both galaxies have experienced a recent star formation burst and show important stellar populations younger than 50 Myr; (3) both galaxies show low surface brightness structures much larger than their main resolved bodies and extending considerably beyond the Holmberg radii; (4) population synthesis based on SBFs shows that the average ages and metallicities of the stellar population of these extended regions are, on average, older than 5 Gyr and more metal-poor than Z=0.0015 for DDO 165 and older than 7 Gyr and more metal-poor than Z=0.0008 for DDO 181; (5) the former results indicate that really old low-metallicity stars probably exist in the extended regions, but nothing can be stated about the presence or not of an intermediate-age population in these regions. The latter is relevant in the characterization of the nature of the extended underlying structures that are being systematically discovered in dIrr galaxies. The question of whether or not these structures are a relic of the primeval star formation activity of the galaxy is fundamental for understanding the formation and early evolution of dwarf galaxies. However, although the SBF population synthesis technique shows promise in the study of unresolved stellar populations, it is not sufficient, in this case, for determining the nature of the extended regions and, in particular, for ascertaining whether they are populated only by very old stars (driving to a two-differentiated-component structure, of the halo/disk kind in spiral galaxies) or whether they are populated by a mixture of old and intermediate-age stars (suggesting a scenario in which the star-forming region of the galaxy is shrinking with time).