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Orbital evidences for dark-matter free Milky-Way dwarf-spheroidal galaxies

Francois Hammer

The nature of Milky Way dwarf spheroidals (MW dSphs) has been questioned, in particular whether they are dominated by dark matter (DM). Here we investigate an alternative scenario, for which tidal shocks are exerted by the MW to DM-free dSphs after a first infall of their gas-rich progenitors, and for which theoretical calculations have been verified by pure N-body simulations. We have shown that the star formation, progenitor histories cannot be sufficiently compelling for discarding one or the other alternative. Advanced precision provided by the GAIA satellite in determining both their past orbital motions and the MW velocity curve is however providing crucial results.

First, tidal shocks explain why DM-free dSphs are found preferentially near their pericenter where they are in a destructive process, while their chance to be long-lived satellites is associated to a very low probability (P< 10−7), which is at odds with the current DM-dominated dSph scenario. Second, most dSph binding energies are consistent with a first infall. Third, the MW tidal shocks that predict the observed dSph velocity dispersions, are themselves predicted in amplitude by the most accurate MW velocity curve. Fourth, tidal shocks accurately predict the forces or accelerations exerted at half light radius of dSphs, including the MW and the Magellanic System gravitational attractions. As the ΛCDM scenario predicts numerous sub-halos, the alternative and more likely hypothesis that is developed here may provoke a significant quake in our understanding of cosmology