Tidal stirring and the origin of dwarf spheroidals in the Local Group

N-body + smoothed particle hydrodynamics (SPH) simulations are used to study the evolution of dwarf irregular galaxies (dIrr's) entering the dark matter halo of the Milky Way or M31 on plunging orbits. We propose a new dynamical mechanism driving the evolution of gas-rich, rotationally supported dIrr's, mostly found at the outskirts of the Local Group (LG), into gas-free, pressure-supported dwarf spheroidals (dSph's) or dwarf ellipticals (dE's), observed to cluster around the two giant spirals. The initial model galaxies are exponential disks embedded in massive dark matter halos and reproduce nearby dIrr's. Repeated tidal shocks at the pericenters of their orbits partially strip their halos and disks and trigger dynamical instabilities that dramatically reshape their stellar components. After only 2-3 orbits low surface brightness dIrr's are transformed into dSp'hs while high surface brightness dIrr's evolve into dE's. This evolutionary mechanism naturally leads to the morphology-density relation observed for LG dwarfs. Dwarfs surrounded by very dense dark matter halos, such as the dIrr GR8, are turned into Draco or Ursa Minor, the faintest and most dark matter dominated among LG dSph's. If disks include a gaseous component, this is both tidally stripped and consumed in periodic bursts of star formation. The resulting star formation histories are in good qualitative agreement with those derived using Hubble Space Telescope (HST) color-magnitude diagrams for local dSph's.