A Comparison of Surface Brightness Profiles for Ultracompact Dwarfs and Dwarf Elliptical Nuclei: Implications for the "Threshing" Scenario

Using imaging from the Hubble Space Telescope, we derive surface brightnessprofiles for ultra-compact dwarfs in the Fornax cluster and for the nuclei ofdwarf elliptical galaxies in the Virgo cluster. Ultra-compact dwarfs are moreextended and have higher surface brightnesses than typical dwarf nuclei, whilethe luminosities, colors, and sizes of the nuclei are closer to those ofGalactic globular clusters. This calls into question the production ofultra-compact dwarfs via ``threshing'' whereby the lower surface brightnessenvelope of a dwarf elliptical is removed by tidal processes, leaving behind abare nucleus. Threshing may still be a viable model if the relatively brightFornax ultra compact dwarfs considered here are descended from dwarfellipticals whose nuclei are at the upper end of their luminosity and sizedistributions.Comment: 5 pages, 2 figures, 1 table, uses emulateapj.cls (included) ApJ Letters, accepted for publicatio