An immense halo stretching over a million miles from its host galaxy, 1,000 times larger than previously suspected, surrounds the Andromeda Galaxy, University of Notre Dame scientists have discovered.
In research published in the Astrophysical Journal, lead investigator Nicolas Lehner and co-investigator J. Christopher Howk, using five years' worth of data from NASA's Hubble Space Telescope, report that they have determined that the halo stretches halfway to our own galaxy, the Milky Way, and the two are expected to collide and form one gigantic galaxy in the future.
Don't hold your breath, though — that collision is expected to occur in about 4 billion years, the scientists report.
Lehner told CBS News,
"Halos are the gaseous atmospheres of galaxies. The properties of these gaseous halos control the rate at which stars form in galaxies, according to models of galaxy formation."
Because the halo is largely invisible, the scientists used quasars, distant star-like bodies, which "shine brightly due to the presence of gas falling onto super-massive black holes in their cores," to measure the size of the halo, Notre Dame reports.
Howk said in the Notre Dame press release, "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. By measuring the dip in brightness, we can tell how much halo gas from M31 [Andromeda] there is between us and that quasar."
Studying the light from 18 quasars, the researchers found that the halo around Andromeda, also known as Messier 31, is indeed massive.
"This is a new milestone because typically only one quasar is used to probe the halos of galaxies beyond the local group," which includes our own galaxy. "Here we have assembled a large sample of quasars that directly demonstrate the true extent of the halo of a single massive galaxy."
Andromeda is 2.5 million light years away from Earth and much brighter, about 25 percent more luminous, than the Milky Way Galaxy, with up to 1 trillion stars, twice as many as those in our own galaxy, HNGN reports.
The halo is about 100 times the size of our full moon, Notre Dame notes. To get a rough sense of its size, hold up two basketballs at arm's length against the backdrop of the sky.
It is likely that the Milky Way also has a halo, although researchers can't be certain. They have discovered halo formations around 44 other galaxies, CBS reports.
The discovery will help astronomers learn about the formation of spiral galaxies, the researchers said.
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