It's well known that the Milky Way is a spiral galaxy, a swirl of stars in an extended, many-armed disk. But the structure of the galaxy is far from two-dimensional. Above and below those familiar spiral arms is a lesser-known feature, a spherical swarm of stars that makes up a halo around the disk.
For decades the presence of the halo has prodded astronomers to ask big questions about its nature: How is it structured? How do stars in the halo compare with disk stars such as our sun, or to stars elsewhere in the halo? And just how did the halo get there? In recent years a group of astronomers has suggested an answer to some of those big questions by drawing on a large telescopic survey of the sky.
The halo, they have concluded, is composed of at least two distinct populations of stars, with different chemical makeups and different orbits. One group of stars, dubbed the inner halo, generally orbits closer to the galactic center, and its members tend to contain more heavy elements such as iron than do stars farther out. (Halo stars as a whole are depleted in these heavy elements, relative to stars in the galactic disk.) Stars of the outer halo occupy somewhat wider orbits around the galactic center, contain lower levels of heavy elements, and—unlike the inner halo—tend to follow retrograde orbits, circling the Milky Way in a direction counter to the rotation of the galactic disk.
"We don't think it's just one halo," says Timothy Beers, an astronomer at the National Optical Astronomy Observatory and Michigan State University, who was lead author on a recent study in The Astrophysical Journal. Beers, Daniela Carollo of Macquarie University in Australia and their colleagues based their analysis on data from the Sloan Digital Sky Survey, a long-running telescopic campaign based at Apache Point Observatory in New Mexico. "We advocate the position that we are looking at a minimum of a dual halo," he says.
As the Milky Way built up by accretion of smaller galaxies, the inner and outer halo would represent two different epochs of galactic assembly. "We actually think that the formation scenario was something you could describe as a multiphase assembly," Beers says. The inner halo would represent the remnants of relatively massive dwarf galaxies, which coalesced early on. Lighter-weight galaxies would have attached themselves later on in a very gradual agglomeration to form the outer halo.
The inner and outer halo are not cleanly divided, but the differences in how the two populations move could aid astronomers in finding extremely primitive stars, which contain primarily hydrogen and helium. Those were the raw materials for the first generation of stars, early in the history of the universe; subsequent generations contained heavier elements that were fused in stellar cores and supernovae and then released into interstellar space. "Knowing that you have this dichotomy helps direct us to finding these interesting low-metallicity stars," Beers says. Outer-halo stars could be identified for detailed study by their distinctive motions on the sky. "Those are the ones that tell the story of how the universe built its elements," Beers says.
But not everyone agrees that the facts support the dual-halo interpretation. "I have a very relaxed opinion about single halos, dual halos, multiple halos," says astrophysicist Ralph Schönrich, a NASA Hubble Fellow at The Ohio State University. "I don't mind any idea of a dual halo. It's just that I don't see any evidence for it."