The spatial and age distribution of stellar populations in DDO 190

The spatial distribution of stellar populations, the star formation history, and other properties of the dwarf irregular (dIrr) galaxy DDO 190 have been analyzed using color-magnitude diagrams of about 3900 resolved stars and the H alpha fluxes of H II regions. From the mean color index of the red giant branch (RGB), a mean metallicity [Fe/H] = -2.0 is obtained. The I magnitude of the tip of the RGB has been used to estimate the distance. DDO 190 is 2.9 +/- 0.2 Mpc from the Milky Way, 2.1 Mpc from the M94 group (Cn 5-1), 2.4 Mpc from the M81 group, and 2.9 Mpc from the barycenter of the Local Group, all indicating that it is an isolated field galaxy. The surface brightness distribution of the galaxy is well fitted by ellipses of ellipticity e = 1 - a/b = 0.1 and position angle of 82 degrees. The radial star density distribution follows an exponential law of scale length alpha = 43. 4, corresponding to 611 pc. The Holmberg semimajor axis to mu(B) = 26.5 is estimated to be r(26.5)(B) = 3.'0. Stellar populations of different ages in DDO 190 show strong spatial decoupling, the oldest population appearing much more extended than the youngest. Stars younger than 0.1 Gyr occupy only the central 40 (0.55 kpc); stars younger than a few (similar to 4) Gyr extend out to similar to 80 (125 kpc), and for larger galactocentric distances, only older stars seem to be present. This behavior is found in all the dIrr galaxies for which spatially extended studies have been performed and could be related to the kinematic history of the galaxy.