Globular clusters—giant balls of hundreds of thousands of tightly spaced, very long-lived stars—are the oldest stellar systems in the Milky Way. Yet after decades of looking for answers, astronomers still don't know how they form. A new study hasn't quite solved the mystery, but it does eliminate one ingredient long thought to be critical: dark matter.
Dark matter is a mysterious, invisible substance, which makes up most of the known universe. Some astronomers believe that, in the early cosmos, it formed halos that compressed gas and dust, sparking the formation of stars. These ancient stars bunched into a spherical formation within the halos, creating the hundreds of globular clusters seen in the Milky Way and other galaxies. Yet evidence linking globular cluster formation with these halos remains elusive.
That may be because dark matter isn't needed, at least according to a new analysis of two globular clusters. In a paper  submitted to The Astrophysical Journal, researchers conclude that there is no evidence of dark matter haloes around NGC 2419, a globular cluster located some 30,000 light-years from the Milky Way's galactic center, and MGC1, a globular cluster located some 65,000 light-years from the center of the nearby Andromeda galaxy.
Harvard University astrophysicist Charlie Conroy and colleagues studied these two particular clusters because they are far from the galactic centers of the Milky way and Andromeda galaxies; that distance has shielded them from cosmic turbulence and kept them—and any putative dark matter—in a relatively pristine state. Using data obtained by other astronomers, the team created computer models of what globular clusters should look like in the presence and absence of dark matter halos. Over time, clusters without dark matter slowly lose their gravitational grip on the stars at their edges, the team found, whereas those with halos hold onto these stars. Both NGC 2419 and MGC1 are missing stars at their fringes, leading the researchers to conclude that they formed in the absence of dark matter halos. The same may be true of most globular clusters in the local universe, says Conroy.
Duncan Forbes, an astronomer at Swinburne University of Technology in Hawthorn, Australia, says the researchers seemed to have proved their case. If dark matter did indeed help form these clusters, he says, it should still be around.
So how do globular clusters form without dark matter? Conroy suspects that violent conditions in the early universe—such as galaxy mergers—shocked and compressed gas and dust in particular areas, creating agglomerations of thousands of stars in particular areas. "One can imagine that old globular clusters formed in the same way stars do now, which has nothing to do with dark matter," says Stacy McGaugh, an astronomer at the University of Maryland, College Park, who was not involved with the study.
To determine whether dark matter is required for any clusters to form, astronomers will need to find clusters that have retained some trace of their parent dark matter halos, says Alexander Muratov, an astronomer at the University of Michigan, Ann Arbor. "To date, this has not been definitively demonstrated by any particular observation."
Knowing for sure how these clusters form is critical to understanding how the universe came to be, says Conroy. "Globular clusters," he says "guide our way toward a more complete understanding of how galaxies assemble and evolve through time."