The Signature of Galactic Tides in Local Group Dwarf Spheroidals

We use N-body simulations to explore the effects of tidal stripping on the structure of dwarf spheroidal galaxies (dSphs). Our models assume cosmologically motivated initial conditions where dSphs are modeled as King spheres, embedded in Navarro-Frenk-White dark halos and orbiting the Galactic potential on eccentric orbits. As expected, systems that orbit through dense regions of the Galaxy lose a significant fraction of stars after each pericentric passage. These episodes of mass loss, however, do not impose a clear tidal cutoff on the bound stellar core. Rather, once equilibrium has been re-established the outer mass profile approaches a power-law well described by a simple Plummer model. As noted in earlier work, tides also result in transient features in the outer density profile. As the system relaxes, an outward-moving “excess” of stars is found at radii where the local crossing time exceeds the time elapsed since pericenter. If the orbit of the dSph is known, these results provide a simple way to assess whether “breaks” and “bumps” in the outer profile of dSphs are actually tidal in origin. We apply this to the Sagittarius dwarf and, encouragingly, identify two features in the surface brightness profile that may be traced to its two last pericentric passages. Applied to Leo I, our results predict that any tidal break would occur at radii beyond those surveyed by current data, casting doubt on recent claims of the detection of tidal debris around this galaxy. For Carina, our model indicates that the tidal break should occur at a radius twice farther than observed. This suggests that the outer excess of stars in Carina is not tidal in origin unless its orbit is in error. A similar comment applies to Sculptor, whose pericenter appears too large for Galactic tides to be important but whose outer profile, like that of Draco, nonetheless follows closely a Plummer-law. Fornax and Leo II show no sign of a power-law outer profile, suggesting that they have not been significantly affected by tides. Published profiles for other Milky Way dSph companions do not extend sufficiently far to allow for conclusive assessment. Panoramic surveys that extend surface brightness profiles beyond ∼ 10 core radii, together with improved constraints on the orbital parameters of dSphs, are needed in order to establish the true origin of the outer envelopes of stars surrounding dSphs. Subject headings: