Free Astronomy Magazine November-December 2020

21 NOVEMBER-DECEMBER 2020 SPACE CHRONICLES nated with this material from stel- lar explosions. The Andromeda gal- axy, also known as M31, is a majestic spiral of perhaps as many as 1 trillion stars and comparable in size to our Milky Way. At a distance of 2.5 mil- lion light-years, it is so close to us that the galaxy appears as a cigar- shaped smudge of light high in the autumn sky. 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. Through a program called Project AMIGA (Absorption Map of Ionized Gas in Andromeda), the study exam- ined 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. Looking through the halo at the quasars’ light, the team observed how this light is ab- sorbed by the Andromeda halo and how that absorption changes in different regions. The immense An- dromeda halo is made of very rari- fied and ionized gas that doesn’t emit radiation that is easily de- tectable. Therefore, tracing the ab- sorption of light coming from a background source is a better way to probe this material. The researchers used the unique ca- pability of Hubble’s Cosmic Origins Spectrograph (COS) to study the ul- traviolet light from the quasars. Ul- traviolet light is absorbed by Earth’s atmosphere, which makes it impos- sible to observe with ground-based telescopes. The team used COS to detect ionized gas from carbon, sili- con and oxygen. An atom becomes ionized when radiation strips one or more electrons from it. Andromeda’s halo has been probed before by Lehner’s team. In 2015, they discovered that the halo is large I n a landmark study, scientists us- ing NASA’s Hubble Space Tele- scope have mapped the immense envelope of gas, called a halo, sur- rounding the Andromeda galaxy, our nearest large galactic neighbor. 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 Androm- eda’s halo is already bumping into the halo of our own galaxy. 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. “Understanding the huge halos of gas surrounding galaxies is im- mensely important,” explained co- investigator Samantha Berek of Yale University in New Haven, Connecti- cut. “This reservoir of gas contains fuel for future star formation within the galaxy, as well as outflows from events such as supernovae. 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 neighbor.” “We find the inner shell that ex- tends to about a half million light- years is far more complex and dynamic ,” explained study leader Nicolas Lehner of the University of Notre Dame in Indiana. “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.” A signature of this activity is the team’s discovery of a large amount of heavy elements in the gaseous halo of Andromeda. Heavier ele- ments are cooked up in the interi- ors of stars and then ejected into space — sometimes violently as a star dies. The halo is then contami-