Hubble probes the invisible halo of a galaxy

Distant quasars serve as distant lighthouse beacons that shine through the gas-rich "fog" of hot plasma encircling galaxies. At ultraviolet wavelengths, Hubble's Cosmic Origins Spectrograph (COS) is sensitive to absorption from many ionised heavy elements, such as nitrogen, oxygen, and neon. COS's high sensitivity allows many galaxies that happen to lie in front of the much more distant quasars to be studied. The ionised heavy elements serve as proxies for estimating how much mass is in a galaxy's halo.