The Milky Way (MW) dwarf spheroidal (dSph) satellites are known to be the
most dark-matter (DM) dominated galaxies with estimates of dark to baryonic
matter reaching even above one hundred. It comes from the assumption that
dwarfs are dynamically supported by their observed velocity dispersions.
However their spatial distributions around the MW is not at random and this
could challenge their origin, previously assumed to be residues of primordial
galaxies accreted by the MW potential. Here we show that alternatively, dSphs
could be the residue of tidal dwarf galaxies (TDGs), which would have
interacted with the Galactic hot gaseous halo and disk. TDGs are gas-rich and
have been formed in a tidal tail produced during an ancient merger event at the
M31 location, and expelled towards the MW. Our simulations show that low-mass
TDGs are fragile to an interaction with the MW disk and halo hot gas. During
the interaction, their stellar content is progressively driven out of
equilibrium and strongly expands, leading to low surface brightness feature and
mimicking high dynamical M/L ratios. Our modeling can reproduce the properties,
including the kinematics, of classical MW dwarfs within the mass range of the
Magellanic Clouds to Draco. An ancient gas-rich merger at the M31 location
could then challenge the currently assumed high content of dark matter in dwarf
galaxies. We propose a simple observational test with the coming GAIA mission,
to follow their expected stellar expansion, which should not be observed within
the current theoretical framework.Comment: 17 pages, 11 figures, accepted by the Monthly Notices of the Royal
Astronomical Society (MNRAS
