Structural Diversity Among the Milky Way's Dwarf Spheroidal Satellites

Abstract

We fit a flexible double power-law (`$\alpha\beta\gamma$') model to the stellar density fields observed for the Milky Way's known dwarf spheroidal satellite galaxies. We show that where standard criteria for model selection are decisive, the $\alpha\beta\gamma$ model is favored over the special case of the Plummer model, and also over the \sersic\ model and its special case, the exponential. The Milky Way's dSph population exhibits a diverse range of stellar density profile shapes, as quantified by the values we infer for outer and inner power-law indices $\beta$ and $\gamma$. Several of the most massive dSphs (e.g., Eridanus II, Fornax, Leo I, Leo II) have steeply-declining outer profiles, with $\beta\gtrsim 8$; others (e.g., Sextans, Bo\"otes I) fade slowly, with $\beta\lesssim 4$. The inner profiles of dSphs with stellar mass $\gtrsim 10^5 M_{\odot}$ are consistent with `cores' of uniform stellar density ($\gamma\approx 0$). At lower masses the slopes of inner density profiles are poorly constrained, except in a few ultrafaint dSphs (e.g., Hercules, Ursa Major II) where we infer steep stellar cusps, with $\gamma\gtrsim 1.5$. Owing to the $\alpha\beta\gamma$ model's flexibility, the inferred halflight radii and total stellar masses are significantly more uncertain than previous estimates, with halflight radii larger by up to an order of magnitude in some cases. Finally, we demonstrate that allowing for flexibility in the shape of the stellar density profile is crucial for dSph mass modeling, where systematic errors associated with choice of stellar density profile can outweigh random errors in the observed velocity dispersions.