Constraining Global Properties of the Draco Dwarf Spheroidal Galaxy

Abstract

By fitting a flexible stellar anisotropy model to the observed surface brightness and line-of-sight velocity dispersion profiles of Draco we derive a sequence of cosmologically plausible two-component (stars + dark matter) models for this galaxy. The models are consistent with all the available observations and can have either cuspy Navarro-Frenk-White or flat-cored dark matter density profiles. The dark matter halos either formed relatively recently (at z ~ 2-7) and are massive (up to ~5 × 109 M☉), or formed before the end of the reionization of the universe (z ~ 7-11) and are less massive (down to ~7 × 107 M☉). Our results thus support either of the two popular solutions of the missing satellites problem of Λ cold dark matter cosmology—that dwarf spheroidals are either very massive or very old. We carry out high-resolution simulations of the evolution of our two-component Draco models in the potential of the Milky Way. The results of our simulations suggest that the observable properties of Draco have not been appreciably affected by the Galactic tides after 10 Gyr of evolution. We rule out Draco being a tidal dwarf—a tidally disrupted dwarf galaxy. Almost radial Draco orbits (with the pericentric distance 15 kpc) are also ruled out by our analysis. The case of a harmonic dark matter core can be consistent with observations only for a very limited choice of Draco orbits (with the apocentric-to-pericentric distances ratio of 2.5).