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 co mpact dwarf galaxy VII Zw 403. VII Zw 403 is the first blue compact dwarf g alaxy for which a clear spatial segregation of the resolved stellar content into a core-halo structure is detected. Multicolor Hubble Space Telescope WFPC2 (HST/WFPC2) observations indicate that active star formation occurs i n the central region, but is strikingly absent at large radii. Instead, a g lobular-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. Whil e the vast majority of blue compact dwarf galaxies have been recognized to possess halos of red color in groundbased surface photometry, our observati ons of VII Zw 403 establish for the first time a direct correspondence betw een 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 popula tions, then the fossil record conflicts with delayed-formation scenarios fo r dwarfs.