F. R. Ferraro/Università di Bologna (main image); Jae-Woo Lee/Sejong University (inset)

Ingested. Globular clusters such as Terzan 5 and M22 (inset) are the remnants of other galaxies swallowed by the Milky Way.

Milky Way Grew by Swallowing Other Galaxies

The motto "E Pluribus Unum" ("out of many, one") could be applied to the Milky Way. Astronomers have obtained new evidence that our home galaxy contains pieces of many former galaxies. The findings strengthen the idea that large galaxies don't emerge whole from single, gigantic clouds of dust and gas. Rather, they grow by swallowing their neighbors.

The clues come from globular clusters--spherical concentrations of up to millions of stars, orbiting the galactic center as self-contained neighborhoods. Aside from our galaxy's huge spiral arms, globular clusters constitute some of its most striking features. Astronomers have long thought they formed from concentrated clouds of gas and dust in the early Milky Way. But two papers in tomorrow's issue of Nature paint an entirely different portrait.

In one paper, a team of Korean astronomers measured the calcium content of stars in 37 of the Milky Way's 158 known globular clusters. About half contained significant amounts of the element, indicating that they had formed from the remnants of supernovae, the titanic explosions of giant stars that once manufactured calcium and other heavy elements in their cores. This finding is significant, says lead author Jae-Woo Lee of Sejong University in Seoul, because it indicates the globular clusters must have once been much larger than they are today. Otherwise, they wouldn't have exerted the gravitational force needed to trap the supernovae remnants so that they could condense into new stars. "This led us to believe that many of the globular clusters we studied are most likely the relics of more massive, primeval dwarf galaxies that merged with [the Milky Way]," he says.

In the second paper, an international team found further evidence for this idea in a globular cluster called Terzan 5. Located above the center of the Milky Way in an area called the galaxy's bulge, the cluster contains an extraordinary number of pulsars, the ultradense, rapidly spinning remnants of supernovae. Chemical analysis revealed that the material ejected in these explosions, rather than escaping from the cluster at high velocity, stuck around to form new stars. This feat required the gravity of a galaxy with up to 1000 times more mass than Terzan 5 currently has. As in the other study, the most likely explanation is that Terzan 5 was once a much larger galaxy that was swallowed by the Milky Way, says lead author Francesco Ferraro of the Università di Bologna in Italy. When Terzan 5 and the other dwarf galaxies merged with the Milky Way, he explains, the collision sheared off many of their stars, leaving the smaller clusters we see today.

Ferraro says further studies of globular clusters should produce more such surprises. "These systems are real gold mines of information," he says, "and each time we observe them they reveal a new, unexpected secret."

The findings could turn previous assumptions about globular clusters on their head, says astronomer David Weinberg of Ohio State University, Columbus. Researchers used to think that one of the Milky Way's clusters, Omega Centauri, was an anomaly because it used to be its own galaxy. Now, Weinberg says, "it looks like anomalous is becoming the new normal."

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