What is the halo around the Milky Way made of? And why is it there? How did it get there?
The halo is a (nearly) spherical region surrounding the galaxy (like the diffuse light around the heads of saints, not like the yellow rings around angels' heads in cartoons), made up mostly of dark matter. Dark matter is called dark because it does not appear to emit any electromagnetic radiation. This means that dark matter can only be studied through its gravitational interactions, in particular, with light passing by, an effect known as "gravitational lensing".
A number of theoretical explanations have been proposed for what dark matter might be made up of, such as "MACHOs" (MAssive Compact Halo Objects) and "WIMPs" (don't blame me, I didn't name them--Weakly Interacting Massive Particles). MACHOs are familiar matter, like dead stars and planets, that were theorized to make up the mass in the halo. However, observations of the rate of gravitational lensing events have ruled out this possibility; there just aren't enough of them. WIMPs are purely theoretical particles and are like neutrinos in that they rarely interact, but unlike neutrinos they can be extremely heavy. The makeup of galactic haloes is important to cosmology, since much of the mass of the universe is in galactic haloes, and how the universe evolves depends strongly on how much of each sort of dark matter there is. It is the subject of enthusiastic study.
Not all of the matter is dark. The halo is home to some ancient star clusters, known as globular clusters. They are known to be old because their stars contain low levels of heavy elements, which weren't present in the young universe and were built up over time in supernovae. The clusters move about the galactic nucleus in (nearly) random orbits.
So how did it get there? The standard model of galaxy formation has all galaxies forming inside dark matter haloes. The dark matter haloes form earlier than the visible structures because due to the fact that dark matter doesn't interact strongly (other than through gravity) it has no pressure support (unlike normal gas). As for the visible matter, the globular clusters likely formed very early on during the formation of our galaxy, while other stars were taken from small galaxies that were disrupted by our galaxy, an idea suggested by some stripes of stars in the halo, like galactic roadkill.
This page was last updated on September 16, 2015