One of the Milky Way's closest galactic neighbors is surrounded by a much bigger halo of gas than previously thought, new observations from the Hubble Space Telescope reveal.
The halo of the Andromeda Galaxy — the nearest spiral galaxy to the Milky Way — is about six times larger and 1,000 times more massive than measured before, the new observations show. It is so large that if the halo were visible from Earth, it would be 100 times the diameter of the full moon — or about the size of two basketballs held at arm's length.
"Halos are the gaseous atmospheres of galaxies," said lead researcher Nicholas Lehner, a physicist at the University of Notre Dame, in Indiana, in a statement from NASA. "The properties of these gaseous halos control the rate at which stars form in galaxies, according to models of galaxy formation." [Amazing Photos of the Andromeda Galaxy]
Scientists spotted the dark halo by looking at bright objects that are behind the gas and seeing how the light changes. Specifically, they used quasars, or galaxies with huge black holes at their cores that are extremely bright despite being far away.
As the light leaves the quasar, some of the gas in the halo absorbs the light and makes the quasar appear darker. This is most apparent when it is observed in ultraviolet light (short wavelengths of light). Measuring the dip in brightness allows astronomers to figure out the amount of gas in front of the quasar.
Because ultraviolet light is absorbed by Earth's atmosphere, the Hubble telescope was ideally placed to see the changes in brightness because it orbits above the planet.
A previous program by Hubble, called the Hubble Cosmic Origins Spectrograph (COS)-Halos program, observed 44 galaxies far away and saw halos. Andromeda, however, is the first close-up galaxy where a halo has been observed.
The authors of the new research say large-scale simulations of galaxies suggest the halo is the same age as the rest of Andromeda, which lies 2.5 million light-years away from Earth. Additionally, the halo is full of elements that are heavier than hydrogen and helium. This indicates at least some of the gas comes from supernovas, explosions of old stars that fused hydrogen and helium into heavier elements through their lifetimes.
As for our own galaxy — the Milky Way — it's possible that it also hosts a halo, but that it's invisible from Earth. "It's a case of not being able to see the forest for the trees," the NASA statement said.. If our galaxy does have a halo, it's possible that it is starting to merge with that of Andromeda's. Our two galaxies are expected to merge into one galaxy starting in roughly 4 billion years.
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Elizabeth Howell (she/her), Ph.D., is a staff writer in the spaceflight channel since 2022 covering diversity, education and gaming as well. She was contributing writer for Space.com for 10 years before joining full-time. Elizabeth's reporting includes multiple exclusives with the White House and Office of the Vice-President of the United States, an exclusive conversation with aspiring space tourist (and NSYNC bassist) Lance Bass, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, flying parabolic, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, "Why Am I Taller?", is co-written with astronaut Dave Williams. Elizabeth holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, a Bachelor of Journalism from Canada's Carleton University and a Bachelor of History from Canada's Athabasca University. Elizabeth is also a post-secondary instructor in communications and science at several institutions since 2015; her experience includes developing and teaching an astronomy course at Canada's Algonquin College (with Indigenous content as well) to more than 1,000 students since 2020. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday. Mastodon: https://qoto.org/@howellspace