Origin of dwarf irregular galaxies with outer early-type structures

Recent observations have reported that some gas-rich dwarf irregular (dIrr) galaxies appear to have spherical distributions in the outer underlying old and intermediate-age stellar populations (e.g., NGC 6822). These observations imply that some dIrr’s have two distinct (or “two-component”) structures, i.e., inner disky and outer spherical ones, though the number fraction of dIrr’s with such structures remains observationally unclear. We discuss how such two distinct structures are formed during dIrr formation based on observations and simulations. Our numerical simulations show that the remnants of mergers between two gas-rich dIrr’s with initially extended gas disks can have both extended spheroids composed of older stellar populations and disks composed mostly of gas and young stars. The simulated remnants with two distinct structures can be still identified as dIrr’s owing to the presence of star-forming regions. The structural properties of outer spherical structures in dIrr’s formed from dIrr-dIrr merging depend on initial conditions of merging, which suggests that outer structures in dIrr’s can be diverse. We also discuss other possible physical mechanisms for the formation of outer spherical structures composed of older stars in dIrr’s. Subject headings: galaxies: irregular – galaxies: ISM – galaxies: dwarf – galaxies:evolution – galaxies:stellar content