FORMATION OF DWARF GALAXIES IN TIDAL TAILS

ZWICKY1 argued long ago that tidal forces can tear long tails of stars and gas from the bodies of interacting disk galaxies, and that this debris may include self-gravitating objects which could become small galaxies. Some recent observations revealing small clumps of stars and gas in tidal tails lend weight to this idea2-4. Here we report the results of numerical simulations of encounters between disk galaxies5, each modelled with a central bulge, an exponential disk and a spheroidal dark-matter halo. We find that dwarf systems form in material drawn out during the encounter; these objects can capture large amounts of moderately enriched gas, but retain little dark matter from their parents' haloes. They should therefore have lower mass-to-light ratios than galaxies formed directly by the collapse of primordial material.