A Stellar Population Gradient in VII Zw 403 - Implications for the Formation of Blue Compact Dwarf Galaxies

We present evidence for the existence of an old stellar halo in the Blue Compact Dwarf galaxy VII Zw 403. VII Zw 403 is the first Blue Compact Dwarf galaxy for which a clear spatial segregation of the resolved stellar content into a "core-halo" structure is detected. Multicolor HST/WFPC2 observations indicate that active star formation occurs in the central region, but is strikingly absent at large radii. Instead, a globular-cluster-like red giant branch suggests the presence of an old (> 10 Gyr) and metal poor (<[Fe/H]>=-1.92) stellar population in the halo. While the vast majority of Blue Compact Dwarf galaxies has been recognized to possess halos of red color in ground-based surface photometry, our observations of VII Zw 403 establish for the first time a direct correspondence between a red halo color and the presence of old, red giant stars. If the halos of Blue Compact Dwarf galaxies are all home to such ancient stellar populations, then the fossil record conflicts with delayed-formation scenarios for dwarfs.