We have uncovered an interesting population of star-forming dwarf galaxies in Local Group-like groups of galaxies. The dwarf galaxies in these groups have very high Halpha equivalent widths, are quite compact, and are mostly not obviously attached to a host galaxy. Integral field spectroscopy with WiFeS reveals that these dwarfs have a wide range of metallicity, some significantly above that expected for their small size. Many also have strong signs of rotation and low mass-to-light ratios measured within 1.5 effective radii.
Such observations indicate that these particular dwarfs are not newly forming in their own cold dark matter haloes, because we would then expect them to have low metallicities and high mass-to-light ratios. Rather, they appear to have formed in clumps of pre-enriched tidal debris flung off of interacting giant galaxies.
We have recently also obtained DEIMOS multi-object spectroscopy to measure rotation curves of the more likely candidates, out to 3 effective radii. Observations of falling rotation curves and low mass-to-light ratios will indicate that these are indeed tidal dwarf galaxies.