ASTROPHYSICS
What is the ‘circumgalactic medium’?
Galaxies are encased in a halo of gas known as the circumgalactic medium
When we think about galaxies, we think about the beautiful pictures of stellar discs that we have seen in the media. They often show grand-design spiral structures with bright, blue stars. Our own galaxy, the Milky Way, is also believed to be a spiral galaxy in which the stars are arranged in a disc-like structure. The galactic disc sits in a much larger and much more massive spherical ‘halo’. The size of the halo is more than ten times the size of the disc, and its mass is about 15 times the mass of the stellar disc. The halo is made of ‘dark matter’ – we do not know what dark matter is, but it’s believed to be made of some subatomic particles.