Shell Structure in the Fornax Dwarf Spheroidal Galaxy

Abstract

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 17' (670 pc in projection) from the center of Fornax. Based on imaging data in both V and I bands down to V ≈ 21, the dimensions of the feature are ~17 × 32 (68 × 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 ≈2 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.