Halo of Gas Around Andromeda Galaxy Mapped for First Time

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Researchers using the Hubble Space Telescope have, for the first time, mapped out the enormous halo of gas around Andromeda, the nearest major galaxy to us. The faint but huge cloud of hot gas stretches out so far that if we could see it with the naked eye, it would cover the width of three Big Dippers.

Although the halo is extremely hard to see, it extends an epic 1.3 light-years away from the galaxy, and up to 2 light-years in some places, stretching almost halfway to the Milky Way. This means, the Hubble scientists say, that the Andromeda halo could be intersecting with the Milky Way’s halo.

M31 Halo Illustration over Rocky Terrain. At a distance of 2.5 million light-years, the majestic spiral Andromeda Galaxy it is so close to us that it appears as a cigar-shaped smudge of light high in the autumn sky. If its gaseous halo could be seen with the naked eye, it would be about three times the width of the Big Dipper — easily the biggest feature on the nighttime sky. NASA , ESA , J. DePasquale and E. Wheatley (STScI ) and Z. Levay

“Understanding the huge halos of gas surrounding galaxies is immensely important,” one of the paper’s authors, Samantha Berek of Yale University, said in a statement. “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.”

The Andromeda Galaxy, also known as M31, is about the same size as the Milky Way and is located 2.5 million light-years away. The researchers were able to map its halo by using Hubble’s Cosmic Origins Spectrograph to look at the light coming from 43 quasars, or distant but very bright galactic nuclei that are located far beyond Andromeda. The light coming from these quasars is scattered by the ionized gas in the halo, allowing the team to map out its reaches.

This illustration shows the location of the 43 quasars scientists used to probe Andromeda’s gaseous halo
This illustration shows the location of the 43 quasars scientists used to probe Andromeda’s gaseous halo. These quasars — the very distant, brilliant cores of active galaxies powered by black holes—are scattered far behind the halo, allowing scientists to probe multiple regions. Looking through the immense halo at the quasars’ light, the team observed how this light is absorbed by the halo and how that absorption changes in different regions. By tracing the absorption of light coming from the background quasars, scientists are able to probe the halo’s material. NASA, ESA, and E. Wheatley (STScI)

Astronomers know that the Milky Way has its own halo, but it’s difficult to study, as we are located within the galaxy. But given the similarities between the Milky Way and Andromeda, we can assume that our own galactic halo has similar properties to the one Hubble studied.

These results therefore teach us more not only about the size and mass of the Andromeda halo, but also about how our own galaxy evolved.

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