The Absence of Extra-Tidal Structure in the Sculptor Dwarf Spheroidal Galaxy

Abstract

The results of a wide-field survey of the Sculptor dwarf spheroidal galaxy are presented. Our aims were to obtain an accurate map of the outer structure of Sculptor and to determine the level of interaction between this system and the Galaxy. Photometry was obtained in two colors down to the magnitude limits of V = 20 and I = 19, covering a 3°.1 × 3°.1 area centered on Sculptor. The resulting color-magnitude data were used as a mask to select candidate horizontal-branch (HB) and red giant branch stars for this system. Previous work has shown that the red HB stars are more concentrated than the blue HB stars. We have determined the radial distributions of these two populations and show that the overall Sculptor density profile is well described by a two-component model based on a combination of these radial distributions. In addition, spectra of the Ca II triplet region were obtained for over 700 candidate red giant stars over the 10 deg2 region using the 2dF instrument on the Anglo-Australian Telescope. These spectra were used to remove foreground Galactic stars based on radial velocity and Ca II triplet strength. The final list of Sculptor members contains 148 stars, 7 of which are located beyond the nominal tidal radius. Both the photometric and spectroscopic data sets indicate no significant extratidal structure. These results support at most a mild level of interaction between this system and the Galaxy, and we have measured an upper mass limit for extratidal material to be 2.3% ± 0.6% of the Sculptor luminous mass. This lack of tidal interaction indicates that previous velocity dispersion measurements (and hence the amount of dark matter detected) in this system are not strongly influenced by the Galactic tidal field.