Galactic tides and the outer density profile of the Sculptor and Ursa Minor dwarf spheroidals

Abstract

Most dwarf spheroidal (dSph) satellites of the Milky Way follow exponential surface density profiles that decline sharply in the outer regions. The Sculptor (Scl) and Ursa Minor (UMi) dSphs deviate from this trend and show a clear excess of stars in the outskirts. Individual members have recently been identified as far as ${\sim}10$ effective radii from the center in both systems. We study whether far-outlying stars in Scl and UMi may result from Galactic tidal forces using idealized N-body simulations. Our results indicate that, on their current orbits, neither galaxy has experienced tidal forces sufficient to affect its stellar density profile. The observed velocity dispersion and size of Scl and UMi imply the dwarfs are simply too dense to have been affected by Galactic tides. We also find weak tidal evolution when including the effects of the Large Magellanic Cloud, which our simulations suggest substantially perturbed Scl's orbit during a close encounter. Our results are insensitive to assumptions about the detailed dark matter density profile of either galaxy, including the presence of an inner core. We conclude that the outlying stars in Scl or UMi are not of tidal origin, but rather innate features that possibly reflect past merger events or the presence of multiple dynamical components.