In a landmark study, scientists using NASA's Hubble Space Telescope have mapped the immense envelope of gas, called a halo, surrounding the Andromeda galaxy, our nearest large galactic neighbour. Scientists were surprised to find that this tenuous, nearly invisible halo of diffuse plasma extends 1.3 million light-years from the galaxy- about halfway to our Milky Way- and as far as 2 million light-years in some directions. This means that Andromeda's halo is already bumping into the halo of our own galaxy.
 
What
  1. They also found that the halo has a layered structure, with two main nested and distinct shells of gas. This is the most comprehensive study of a halo surrounding a galaxy.
  2. Understanding the huge halos of gas surrounding galaxies is immensely important, said co-investigator Samantha Berek of Yale University in New Haven, Connecticut.
  3. This reservoir of gas contains fuel for future star formation within the galaxy, as well as outflows from events such as supernovae. 
  4. It's full of clues regarding the past and future evolution of the galaxy, and we're finally able to study it in great detail in our closest galactic neighbour.
  5. The outer shell is smoother and hotter. This difference is a likely result from the impact of supernova activity in the galaxy's disk more directly affecting the inner halo.
  6. A signature of this activity is the team's discovery of a large amount of heavy elements in the gaseous halo of Andromeda
  7. Heavier elements are cooked up in the interiors of stars and then ejected into space -- sometimes violently as a star dies. The halo is then contaminated with this material from stellar explosions.
  8. The Andromeda galaxy, also known as M31, is a majestic spiral of perhaps as many as 1 trillion stars and comparable in size to our Milky Way. 
  9. At a distance of 2.5 million light-years, it is so close to us that the galaxy appears as a cigar-shaped smudge of light high in the autumn sky. 
  10. If its gaseous halo could be viewed with the naked eye, it would be about three times the width of the Big Dipper. This would easily be the biggest feature on the nighttime sky.
  11. Through a program called Project AMIGA (Absorption Map of Ionized Gas in Andromeda), the study examined the light from 43 quasars -- the very distant, brilliant cores of active galaxies powered by black holes -- located far beyond Andromeda. The quasars are scattered behind the halo, allowing scientists to probe multiple regions.
  12. The immense Andromeda halo is made of very rarified and ionized gas that doesn't emit radiation that is easily detectable. Therefore, tracing the absorption of light coming from a background source is a better way to probe this material.
  13. The researchers used the unique capability of Hubble's Cosmic Origins Spectrograph (COS) to study the ultraviolet light from the quasars. 
  14. Ultraviolet light is absorbed by Earth's atmosphere, which makes it impossible to observe with ground-based telescopes. The team used COS to detect ionized gas from carbon, silicon and oxygen. An atom becomes ionized when radiation strips one or more electrons from it.
  15. Andromeda's halo has been probed before by Lehner's team. In 2015, they discovered that the Andromeda halo is large and massive. But there was little hint of its complexity; now, it's mapped out in more detail, leading to its size and mass being far more accurately determined.
  16. In fact, Andromeda is the only galaxy in the universe for which this experiment can be done now, and only with Hubble. Only with an ultraviolet-sensitive future space telescope will scientists be able to routinely undertake this type of experiment beyond the approximately 30 galaxies comprising the Local Group.