A Wide Field Planetary Camera 2 Study of the Resolved Stellar Population of the Pegasus Dwarf Irregular Galaxy (DDO 216)

The stellar population of the Pegasus dwarf irregular galaxy is investigatedin images taken in the F439W (B), F555W (V), and F814W (I) bands with WFPC2.These and ground-based data are combined to produce color-magnitude diagramswhich show the complex nature of the stellar population in this small galaxy. Ayoung (< 0.5 Gyr) main sequence stellar component is present and clustered intwo centrally-located clumps, while older stars form a more extended disk orhalo. The colors of the main sequence require a relatively large extinction ofA_V = 0.47 mag. The mean color of the well-populated red giant branch isrelatively blue, consistent with a moderate metallicity young, or older,metal-poor stellar population. The red giant branch also has significant widthin color, implying a range of stellar ages and/or metallicities. A small numberof extended asymptotic giant branch stars are found beyond the red giant branchtip. Near the faint limits of our data is a populous red clump superimposed onthe red giant branch. Efforts to fit self-consistent stellar population modelsbased on the Geneva stellar evolution tracks yield a revised distance of 760kpc. Quantitative fits to the stellar population are explored as a means toconstrain the star formation history. The numbers of main sequence and corehelium-burning blue loop stars require that the star formation rate was higherin the recent past, by a factor of 3-4 about 1 Gyr ago. Unique results cannotbe obtained for the star formation history over longer time baselines withoutbetter information on stellar metallicities and deeper photometry. The youngestmodel consistent with the data contains stars with constant metallicity of Z =0.001 which mainly formed 2-4 Gyr ago. Even at its peak of star formingactivity, the Pegasus dwarf most likely remained relatively dim with M_V ~ -14.Comment: 46 pages, 16 figures, 1 tabl