The Circumgalactic Medium

Galaxies have been traditionally observed via the optical light emitted by stars; however, a substantial amount of gas lies outside the optical disk of the galaxy.  This gas is still bound to the galaxy and is difficult to observe, but can be detected via absorption line studies (e.g. HI, MgII, OVI, CIV), and may soon be observed in emission lines such as Lya (the image to the right is the predicted Lya emissivity in a region 500 kpc on a side of a star bursting galaxy)).

The Intergalactic Medium

Gas also exists outside of galaxies, both in a low temperature form (the Lyman alpha forest), and at high temperature, known as the Warm-Hot Ionized Medium (WHIM).  The image to the right shows the distribution of gas in a box of size 20 Mpc/h.