The Milky Way's third population.

The Milky Way's third population

The Milky Way galaxy consists of a thin disk of stars with a central bulge surrounded by a halo of stars in globular clusters. The stars in the disk are mostly young stars rich in heavy elements, mainly metals. The globular clusters in the halo contain old stars with low concentrations of heavy elements.

Several years ago, a number of astronomers suggested our galaxy may contain a third distinctive population of stars. A team of astronomers has now collected evidence supporting the idea. They suggest this third population may have resulted from a merger of separate bodies of stars early in the galaxy's history.

Data gathered by Bruce W. Carney of the University of North Carolina in Chapel Hill and his collaborators show stars that in their sample appear to fall into three categories, characterized by differences in their heavy-element content. The concentration of heavy elements in the third population falls between the high concentration in thin-disk stars and the low concentration in halo stars. The researchers contend that stars in this newly identified third population, which they call the "thick disk," are old and formed over a short period of time.

Such thick-disk stars could have resulted from the merger of our galaxy with another large clump of matter shortly after the thin disk formed. The heating of the galactic disk resulting from the merger could have driven both stars and gas to greater heights above the disk's plane, resulting in the formation of stars with peculiar characteristics. The stars driven out of the thin disk by the collision and those formed from gas at such heights would remain there and would make up the stellar component now termed the thick disk.

"If this is the cause of our galaxy's thick disk, and if other galaxies...show signs of such components..., it is conceivable that mergers play a major role in galaxy formation, and perhaps help determine the type of galaxy that eventually results," the researchers conclude in the February ASTRONOMICAL JOURNAL.