Galactic Halo
The galactic halo is the most extensive component of a galaxy. It is a spherical, diffuse region that envelops the galactic disk and bulge. It contains very old stars, globular clusters, and, for the most part, dark matter. In the case of the Milky Way, the halo extends over several hundred thousand light-years.
Composition of the halo
The Milky Way’s stellar halo contains relatively few stars compared to the disk, but these are among the oldest in the galaxy—some are more than 13 billion years old. They are poor in heavy elements because they formed before several generations of supernovae enriched the interstellar medium. Globular clusters also orbit within the halo, forming a sort of crown around the galaxy.
The dark matter halo
The dominant component of the halo is not stellar but consists of dark matter. This halo of dark matter, invisible but gravitationally active, is essential for explaining the Milky Way’s rotation curve: the stars at the edge of the disk orbit too fast for the amount of visible matter present. The mass of the dark matter halo is thought to be several times greater than the total mass of all the stars in the galaxy.
Stellar streams
The study of the halo reveals the history of past mergers in the Milky Way. Absorbed dwarf galaxies have left behind stellar streams— long streams of stars orbiting in the halo — that allow us to reconstruct the history of our galaxy. The Sagittarius Stream, produced by the Sagittarius dwarf galaxy currently being absorbed, is one of the most spectacular.
Did you know?
The Milky Way’s galactic halo likely extends up to 300,000 light-years from the galactic center, well beyond the visible disk (which is about 100,000 light-years in diameter). The exact boundary between our halo and that of Andromeda is still a matter of debate.
Frequently Asked Questions
Can the galactic halo be observed?
The stellar halo is very diffuse and difficult to observe directly. It is mainly detected through the globular clusters that inhabit it, halo stars identifiable by their low metallicity, and stellar streams mapped by missions such as Gaia.
Do all galaxies have a halo?
Yes, all large galaxies appear to have a halo of dark matter. The stellar halo is more developed in galaxies that have undergone numerous mergers. Dwarf galaxies also have dark matter halos, which are proportionally even larger than their visible component.
What is the difference between the halo and the galactic bulge?
The bulge is the dense, spheroidal central region of the galaxy, located at the heart of the disk. The halo is the extensive spherical region that envelops the entire galaxy—both the disk and the bulge. The bulge contains old stars as well as gas and residual star formation, while the halo is much more diffuse and devoid of cold gas.