Identifying old tidal dwarf irregulars

We examine the observational consequences of the two possible origins for i rregular galaxies: formation from collapse of a primordial cloud of gas ear ly in the age of the universe, or formation from tidal tails in an interact ion that could have occurred any time in the history of the universe. Becau se the formation from tidal tails could have occurred a long time ago, prox imity to larger galaxies is not sufficient to distinguish tidal dwarfs from traditional dwarfs. We consider the effects of little or no dark matter on rotation speeds and the Tully-Fisher relationship, the metallicity-luminos ity relationship, structure, and stellar populations. From these selection criteria, we identify a small list of dwarf irregular galaxies that are can didates for having formed as tidal dwarfs.