Tidal tails tell tales of newborn galaxies

Talk about big-time mergers. In the infant universe, marauding galaxies devoured each other, coalescing to form bigger galaxies. These hostile takeovers spewed out truly hot commodities—gas and dust that could become the ingredients for nascent stars.

The tug of war between two galaxies (left) has created two tidal tails. Inset shows the lower tail, indicating locations of atomic hydrogen (white contour lines) and carbon monoxide (faint red circle). Braine et al./Nature

A similar process may still be unfolding. Whenever two galaxies come close enough, their mutual gravity rips long streamers of gas and dust from each other. Some of these so-called tidal tails may become galaxies in their own right, undertaking the same evolution that their ancestors did billions of years ago.

A new study adds to the evidence that some tails containing young stars are indeed small galaxies in the making. Studying two tails torn from different pairs of mature galaxies a few hundred million light-years from Earth, Jonathan Braine of the Bordeaux Observatory in France and his colleagues argue that both tails cook up their own molecular hydrogen—the raw material for making stars—rather than steal the stuff from their parents. The researchers describe their observations in the Feb. 24 Nature.

If the results hold, they “would offer astronomers the exciting prospect of studying up close a potentially important galaxy-formation process,” says Gary Welch of Saint Mary’s University in Halifax, Nova Scotia.

Braine and his colleagues examined the tails with the Institut de Radioastronomie Millimétrique’s 30-meter radio telescope atop Pico Veleta in Spain. Because molecular hydrogen emits little radiation, they traced the molecule by hunting for emissions from a companion molecule, carbon monoxide. In both tails, the team found the highest concentrations of molecular hydrogen where densities of atomic hydrogen were highest.

This suggests that most of the molecular hydrogen was not simply torn off the parent galaxies, as the atomic hydrogen was, because the molecular and atomic forms typically have very different distributions in such galaxies. Molecular hydrogen tends to cluster at the center of a mature spiral galaxy, whereas atomic hydrogen spreads out.

The observations suggest that the tails are making fresh molecular hydrogen from atomic hydrogen, Braine and his colleagues argue. “Star formation in the tidal dwarf galaxies therefore appears to mimic the process in normal spiral galaxies like our own,” the researchers say.

Although other astronomers have detected carbon monoxide in tidal tails, those tails didn’t appear to be making stars. “Until this paper came out, there wasn’t any clear detection of molecular gas in something that looked like it might be forming a galaxy,” says Welch.

Rosemary F.G. Wyse of Johns Hopkins University in Baltimore says that dwarf galaxies formed from tidal tails would have a proportion of dark matter—the unseen material that makes up 90 percent of the mass of the universe—similar to that in large, spiral galaxies like the Milky Way. That would distinguish them from the many dwarf galaxies known to have a much greater dark matter content.

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