When galaxies try to cozy up, things can get nasty. Small contenders are cannibalized, and more equally matched competitors often tear each other to shreds before sashaying into eternal togetherness.

All this gravity-induced carnage, which astronomers dryly call tidal interaction, can create curious tidal tails -- long strands of stars and gas that break off and stream out into space behind the colliding parents.

There, in these tails, tidal dwarf galaxies are born, and new stars appear to emerge.

Tidal dwarf galaxies were first discussed in 1992, based on hints of their existence dating back to 1956. But until now researchers have not been able to figure out how these offspring get their molecular hydrogen, the stuff of star formation. Were clouds of molecular hydrogen left behind by the parents? Or did the galactic infants produce it themselves?

New evidence suggests tidal dwarf galaxies do produce their own molecular hydrogen -- just like their parents are thought to have done. The finding, reported in the February 24 issue of the journal Nature, is expected to improve overall understanding of galaxy formation.

"Star formation in the tidal dwarf galaxiesappears to mimic the process in normal spiral galaxies like our own," write the paper's authors, led by Jonathan Braine of the Observatoire de Bordeaux, France.

The finding, if it holds up, implies that as the universe evolves, new galaxies are born from leftover debris, says Gary Welch of Saint Mary's University in Canada.

"The gas in the tidal dwarf galaxies was once in a larger galaxy, where it might one day have been used to make stars. Now, perhaps, some of it -- but only a small part -- will be used to make stars in a different galaxy," said Welch, who was not involved in the research.

"The finding about tidal dwarf galaxies tells us that galaxy interactions are more interesting and complex than we realized."

The process is not the same as common stellar recycling, Welch said, because the gas is not so much re-used; rather it is left over from a previous round of star formation, which is known to be an inefficient process.

"The nearest thing to true recycling is when an old star blasts its guts back out into space," Welch told SPACE.com. "This gas mixes with what's already there, then later stars form from it. This is a common process in our galaxy and others."

Welch said the newly discovered process might be responsible for some of the abundant dwarf galaxies found in the universe. The question, he said, is how many.

"The finding about tidal dwarf galaxies tells us that galaxy interactions are more interesting and complex than we realized," Welch said.

Molecular hydrogen does not radiate much energy, so Braine and his colleagues inferred its presence by detecting associated trace molecules, primarily carbon monoxide, in two tidal dwarf galaxies. Previous studies had also detected carbon monoxide in tidal dwarf galaxies, but the results were inconclusive, said Welch.

In the same region where the molecular hydrogen was found, the researchers spotted atomic hydrogen. This implies that the dwarf galaxy itself has reprocessed some of the atomic hydrogen into molecular hydrogen, which is then ready for star formation.

This system of two interacting galaxies is known as the Guitar. A tidal dwarf galaxy has formed at the far right, and another at the far left. Carbon monoxide was found around the center of the inset image.

Welch said the limited number of sample areas within the tidal dwarf galaxies suggests that further study would improve confidence in the results.

"I think [the results are] reasonably good, but not airtight," he said.