Credit: Todd M. Tripp, UMass Amherst and NASA Galaxy Evolution Explorer
The illustration shows how quasistellar object (QSO) absorption lines are used to study the vast (and effectively invisible) gaseous halos of galaxies. When the light from a distant QSO passes through the gas surrounding a foreground galaxy (schematically indicated with a red dashed circle), some of the colors of the QSO light are absorbed by the foreground material. Consequently, the Hubble Space Telescope observes that some of the colors are “missing.” By studying the absorbed colors, astronomers can determine many things about the gaseous halo, such as composition, temperature, density and mass. This technique has revealed that the gaseous halos of galaxies as much larger and more massive than the distribution of stars within the galaxy. These large halos are produced by “winds” of matter rapidly moving away from the galaxies.