EXPLORING HALO SUBSTRUCTURE WITH GIANT STARS. VIII. THE EXTENDED STRUCTURE OF THE SCULPTOR DWARF SPHEROIDAL GALAXY

We explore the spatial distribution of stars in the Sculptor dwarf spheroidal (dSph) galaxy over an area of 7.82 deg2, including coverage of the central region but extending mostly south and east of the dSph core. Two methods are used to identify stars that are most likely associated with the dSph, and these filtered samples of stars are used to map its spatial structure. First, following the method of previous contributions in this series, we utilize Washington M, T2 + DDO51 photometry to identify red giant branch (RGB) star candidates with approximately the same distance and metallicity as the Sculptor dSph. Second, a prominent blue horizontal branch (BHB) population provides a fairly populous and pure sample of Sculptor stars having broadband colors unlike the bulk of the Galactic field star population. A spectroscopically observed subset of Sculptor candidate stars (147 total stars: ~5% of all Sculptor candidates and ~10% of Sculptor giant candidates) yields a systemic heliocentric velocity for the system of vhel = 110.43 ± 0.79 km s-1, in good agreement with previous studies. We also find a global velocity dispersion of σv = 8.8 ± 0.6 km s-1, with slight indications of a rise in the velocity dispersion past ~0.4rlim. These spectra also provide a check on the reliability of our candidate Sculptor giant sample to M ~ 19; 94% of the photometrically selected Sculptor giant star candidates with follow-up spectroscopy are found to be kinematically associated with Sculptor, while 4 out of 10 stars outside of our Sculptor giant star selection criteria that we tested spectroscopically appear to be velocity members of Sculptor. These percentages are in agreement with results for an additional 22 Sculptor field stars with radial velocities in the literature. All available velocities show that our methodology for picking Sculptor giants is both reliable and conservative. Thus, these giant star samples should provide a reliable means to explore the structure of the Sculptor dSph. Nevertheless, considerable care has been taken to assess the level of background contamination in our photometric sample to ensure an accurately derived density profile of the Sculptor dSph to large radii. Multiple background assessments verify that we detect a considerable stellar density of Sculptor stars to the limits of our main survey area for both the RGB and BHB candidate samples. While we find that a King profile of limiting radius rlim = 796 fits the density profile of Sculptor well to ~60', beyond this, we identify a "break" in the profile and a clearly detected population of Sculptor stars following a Σ ∝ r-2 decline to more than 2rlim. This break population must signify either the presence of an extremely broad distribution of bound "halo stars" around the Sculptor dSph or the presence of unbound tidal debris. If the latter is true, we determine a fractional mass-loss rate of approximately 0.042 Gyr-1 for the Sculptor dSph. Additional support for the notion that there is tidal disruption comes from the two-dimensional distribution of our Sculptor candidate stars; both the RGB and BHB samples show increasingly elongated isodensity contours with radius that point to an apparent stretching reminiscent of what is seen in models of disrupting satellite galaxies. Finally, we find that RGB stars that are more likely to be metal-poor (based on their color and magnitude) are significantly less centrally concentrated and therefore constitute the primary contributing stellar population to the likely tidally stripped parts of the dSph.