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 BlueCompact Dwarf galaxy VII Zw 403. VII Zw 403 is the first Blue Compact Dwarfgalaxy for which a clear spatial segregation of the resolved stellar contentinto a "core-halo" structure is detected. Multicolor HST/WFPC2 observationsindicate that active star formation occurs in the central region, but isstrikingly absent at large radii. Instead, a globular-cluster-like red giantbranch suggests the presence of an old (> 10 Gyr) and metal poor(<[Fe/H]>=-1.92) stellar population in the halo. While the vast majority ofBlue Compact Dwarf galaxies has been recognized to possess halos of red colorin ground-based surface photometry, our observations of VII Zw 403 establishfor the first time a direct correspondence between a red halo color and thepresence of old, red giant stars. If the halos of Blue Compact Dwarf galaxiesare all home to such ancient stellar populations, then the fossil recordconflicts with delayed-formation scenarios for dwarfs.Comment: Accepted for publication in the Ap