Formation of molecular gas in the tidal debris of violent galaxy-galaxy interactions

In many gravitational interactions between galaxies, gas and stars that hav e been torn from the precursor galaxies can collect in tidal 'tails'. Star formation begins anew in some of these regions, producing tidal dwarf galax ies(1-4). Observations of these new galaxies provides insight into processe s relevant to galaxy formation more generally, because the timescale of the interaction is well defined. But tracking the star formation process has h itherto been difficult because the tidal dwarf galaxies with young stars sh owed no evidence of the molecular gas out of which those young stars formed (5-8). Here we report the discovery of molecular hydrogen (traced by carbon monoxide emission) in two tidal dwarf galaxies. In both cases, the concent ration of molecular gas peaks at the same location as the maximum in atomic -hydrogen density, unlike the situation in most gas-rich galaxies. We infer from this that the molecular gas formed from the atomic hydrogen, rather t han being torn in molecular form from the interacting galaxies. Star format ion in the tidal dwarf galaxies therefore appears to mimic the process in n ormal spiral galaxies like our own.