A Wide-Field Survey of the Fornax Dwarf Spheroidal Galaxy

The results of a photometric survey of the Fornax dwarf spheroidal galaxy are presented herein. Accurate photometry in two colors (V and I) was collected over a 10 deg² area centered on the Fornax system with the aim of searching for extratidal structure. The data were complete to a magnitude of V=20, or approximately 1 mag brighter than the Fornax red clump stars. Stars were selected with a color and magnitude lying near the Fornax red giant branch, thereby reducing contamination from the field-star population. We were thus able to probe the outer structure of this dwarf galaxy. Immediately visible was a shell-like structure located 1.3d northwest from the center of Fornax, approximately 30' beyond the nominal tidal radius at this position angle. We have measured the absolute visual magnitude of this feature to be M_V~-7. The feature is aligned with a previously reported shell (age~2 Gyr) located near the core radius of Fornax. A statistical analysis of the extratidal stellar distribution further revealed two lobes situated on the Fornax minor axis that are aligned with the two shell-like features. The two-lobed structure combined with the two shells provide strong evidence that Fornax has experienced a merger event in the relatively recent past.