Though the merger of the Milky Way and Andromeda galaxy isn’t expected to happen for another 4 billion years, the recent discovery of a massive halo of hot, ionized gas around Andromeda may mean our galaxies are already touching. Using the Hubble Space Telescope, astrophysicist Nicholas Lehner and a team of scientists were able to identify halo at least 2 million light-years in diameter surrounding the galaxy.

The Andromeda Galaxy is the largest in a collection of about 54 galaxies in the Local Group, which includes the Milky Way. With twice as many stars as the Milky Way, it shines 25% brighter and can be seen with the naked eye from rural skies. If the halo extends a million light years in our direction, our two galaxies are much closer to touching than previously thought. We may even be mingling molecules.

Lehner describes the halos as the “gaseous atmospheres of galaxies.” To find and study the nearly-invisible halo, the team sought out quasars. The brightest quasar, 3C273 in Virgo, can be seen in a 6-inch telescope. Their pinpoint nature makes them the perfect probes for the job.

“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,” J. Christopher Howk, an associate professor of physics at Notre Dame, told reporters. “By measuring the dip in brightness, we can tell how much halo gas from M31 there is between us and that quasar.”

Astronomers have observed halos around 44 other galaxies, but never one as massive as Andromeda, where so many quasars are available to clearly define its extent. The previous 44 were all extremely distant galaxies, with only a single quasar to determine halo size and structure.

The halo is estimated to contain half the mass of the stars in the Andromeda galaxy itself, in the form of hot diffuse gas. Although it is mostly composed of ionized hydrogen, Andromeda’s aura is rich in heavier elements that likely come from supernovae. They erupt within the galaxy and blow iron, silicon, oxygen and other elements into space. Nearly half of all the heavy elements made by Andromeda’s stars have been expelled beyond the galaxy’s 200,000-light-year-diameter stellar disk.

The next time you look up and see Andromeda, just remember, it’s closer than you think!

Source: Universe Today


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