A Wide-Field Survey of the Fornax Dwarf Spheroidal Galaxy

Abstract

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 Fomax 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°.3 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 MV ∼ -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.