Free Astronomy Magazine July-August 2015

SPACE CHRONICLES never before has such a massive halo been seen in a neighboring galaxy. Because the previously studied gal- axies were much farther away, they appeared much smaller on the sky. Only one quasar could be detected behind each faraway galaxy, provi- ding only one light anchor point to map their halo size and structure. With its close proximity to Earth and its correspondingly large footprint on the sky, Andromeda provides a far more extensive sampling of a lot of background quasars. "As the light from the quasars travels toward Hubble, the halo's gas will absorb some of that light and make the quasar appear a little darker in just a very small wavelength range," ex- plains co-investigator J. Christopher Howk, also of Notre Dame. "By measuring the dip in brightness in that range, we can tell how much halo gas from M31 there is between us and that quasar." The scientists used Hubble's unique capability to study the ultravio- let light from the quasars. Ultraviolet light is absorbed by Earth's atmosphere, which makes it dif- ficult to observe with a ground-bas- ed telescope. The team drew from about 5 years' worth of observa- tions stored in the Hubble data archive to conduct this re- search. Many pre- vious Hubble cam- paigns have used quasars to study gas much farther away than — but in the general direction of — Andromeda, so a treasure trove of da- ta already existed. But where did the giant halo come from? Large- scale simulations of galaxies sug- gest that the halo formed at the same time as the rest of Andro- meda. The team also determined that it is enriched in elements much heavier than hydrogen and helium, and the only way to get these heavy elements is from exploding stars called supernovae. The super- novae erupt in Andromeda's star- filled disk and violently blow these heavier elements far out into space. Over Andromeda's lifetime, nearly half of all the heavy elements made by its stars have been expelled far beyond the galaxy's 200,000-light- year-diameter stellar disk. What does this mean for our own galaxy? Because we live inside the Milky Way, scientists cannot deter- mine whether or not such an equally massive and extended halo exists around our galaxy. It's a case of not being able to see the forest for the trees. If the Milky Way does possess a similarly huge halo, the two galaxies' halos may be nearly touching already and quies- cently merging long before the two massive galaxies collide. Hubble observations indicate that the Andromeda and Milky Way gal- axies will merge to form a giant el- liptical galaxy beginning about 4 billion years from now. T his diagram shows how scientists determined the size of the halo of the Andromeda galaxy. Because the gas in the halo is dark, the team measured it by using the light from quasars, the very distant bright cores of active gal- axies powered by black holes. They observed the quasars' light as it traveled through the intervening gas. The halo's gas absorbed some of that light and made the quasar ap- pear darker in a very small wavelength range. By measur- ing the tiny dip in brightness at that specific range, scientists could tell how much gas is between us and each quasar. Some quasars showed no dip in brightness, and this helped define the size of the halo. [NASA, ESA, A. Feild (STScI), N. Lehner and J.C. Howk (University of Notre Dame), and B. Wakker (University of Wisconsin, Madison)] n