Shell structure in the Fornax dwarf spheroidal galaxy

We present initial results from a wide-field photometric survey of the Fornax dwarf spheroidal (dSph) galaxy. The focus here is on a small overdensity of stars located approximately 170 ( 670 pc in projection) from the center of Fornax. Based on imaging data in both V and I bands down to V approximate to 21, the dimensions of the feature are similar to1.'7 x 3.'2 (68 x 128 pc) with an average surface brightness of 25.8 mag arcsec(-2) in V. A follow-up analysis using the deep B- and R-band data obtained by Stetson, Hesser, & Smecker-Hane indicates that the overdensity is apparently dominated by a relatively young stellar population with an age of approximate to2 Gyr, though the current data do not rule out the presence of older stars. Our preferred interpretation is that the overdensity represents shell structure in the Fornax dSph galaxy, a phenomenon previously unseen in dwarf galaxies. The shell may be the remnant of a merger with Fornax of a smaller, gas-rich system that occurred approximately 2 Gyr ago. The comparatively recent star formation seen in some dSph and dE galaxies may then also be the result of infall of small relatively gas-rich systems.