Andromeda Galaxy's Halo is 1000 Times More Massive Than Previously Thought

Scientists using NASA's Hubble Space Telescope have discovered that the immense halo of gas enveloping the Andromeda galaxy is about six times larger and 1,000 times more massive than previously thought. This dark halo stretches about half a million light-years from its host galaxy to our own Milky Way, revealing a bit more about the evolution and structure of giant spirals.

"Halos are the gaseous atmospheres of galaxies," said Nicolas Lehner, one of the researchers, in a news release. "The properties of these gaseous halos control the rate at which stars form in galaxies according to models of galaxy formation."

The Andromeda galaxy lies 2.5 million light-years away and looks a bit like a faint spindle that's 6 times the diameter of the full moon. It's considered to be a near-twin to the Milky Way galaxy.

In order to get a better glimpse at Andromeda's halo, the researchers looked at bright background objects through the gas and observed how the light changed. This is a bit like looking at a glowing light at the bottom of a pool at night. The ideal background lights for this kind of effort are quasars, which are the distant, bright cores of active galaxies.

"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," said J. Christopher Howk, co-investigator of the new study. "By measuring the dip in brightness in that range, we can tell how much halo gas from Andromeda there is between us and that quasar."

The researchers used Hubble to study the ultraviolet light from the quasars. In the end, they found that the Andromeda galaxy had a massive halo.

But where did the halo come from? The researchers believe that it formed at around the same time as the rest of Andromeda. It's rich in elements much heavier than hydrogen and helium, and the only way to get these heavy elements is exploding stars called supernovae.

Currently, researchers aren't sure whether our Milky Way has a similar halo. However, Andromeda's halo does suggest that this may be the case.

The findings are published in The Astrophysical Journal.

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