Tidal Stirring and the Origin of Dwarf Spheroidals in the Local Group

N-Body/SPH simulations are used to study the evolution of dwarf irregulargalaxies (dIrrs) entering the dark matter halo of the Milky Way or M31 onplunging orbits. We propose a new dynamical mechanism driving the evolution ofgas rich, rotationally supported dIrrs, mostly found at the outskirts of theLocal Group (LG), into gas free, pressure supported dwarf spheroidals (dSphs)or dwarf ellipticals (dEs), observed to cluster around the two giant spirals.The initial model galaxies are exponential disks embedded in massive darkmatter halos and reproduce nearby dIrrs. Repeated tidal shocks at thepericenter of their orbit partially strip their halo and disk and triggerdynamical instabilities that dramatically reshape their stellar component.After only 2-3 orbits low surface brightness (LSB) dIrrs are transformed intodSphs, while high surface brightness (HSB) dIrrs evolve into dEs. Thisevolutionary mechanism naturally leads to the morphology-density relationobserved for LG dwarfs. Dwarfs surrounded by very dense dark matter halos, likethe archetypical dIrr GR8, are turned into Draco or Ursa Minor, the faintestand most dark matter dominated among LG dSphs. If disks include a gaseouscomponent, this is both tidally stripped and consumed in periodic bursts ofstar formation. The resulting star formation histories are in good qualitativeagreement with those derived using HST color-magnitude diagrams for localdSphs.