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 investigated in images taken in the F439W (B), F555W (V), and F814W (I) bands with the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope. With WFPC2 the Pegasus dwarf is highly resolved into individual stars to limiting magnitudes of about 25.5 in B and V and 25 in I. These and ground-based data are combined to produce color-magnitude diagrams that show the complex nature of the stellar population in this small galaxy. A young (<0.5 Gyr) main-sequence stellar component is present and clustered in two centrally located clumps, while older stars form a more extended disk or halo. The colors of the main sequence require a relatively large extinction of A(V) = 0.47 mag. The mean color of the well-populated red giant branch (RGB) is relatively blue, consistent with a moderate-metallicity young, or older metal-poor, stellar population. The RGB also has significant width in color, implying a range of stellar ages and/or metallicities. A small number of extended asymptotic giant branch stars are found beyond the RGB tip. Near the faint limits of our data is a populous red clump superposed on the RGB. Efforts to fit self-consistent stellar population models based on the Geneva stellar evolutionary tracks yield a revised distance of 760 kpc. Quantitative fits to the stellar population are explored as a means of constraining the star formation history. The numbers of main-sequence and core helium burning blue-loop stars require that the star formation rate was higher in the recent past, by a factor of 3-4 about 1 Gyr ago. Unique results cannot be obtained for the star formation history over longer time baselines without better information on stellar metallicities and deeper photometry. The youngest model consistent with the data contains stars with constant metallicity of Z = 0.001 that mainly formed 2-4 Gyr ago. If stellar metallicity declines with increasing stellar age, then older ages are allowed of up to approximate to 8 Gyr. However, even at its peak of star-forming activity, the intermediate-age-dominated model for the Pegasus dwarf most likely remained relatively dim, with M-V approximate to -14.