Andromeda's Massive Halo Indicates Its Merger With Milky Way Has Begun

Scientists have discovered that the halo around Andromeda is bigger than they earlier thought.

Using NASA's Hubble Space Telescope, researchers studied light from distant objects beyond Andromeda, Milky Way's closest neighbor to learn that its halo is about thousand times more massive and six times larger than earlier estimates as the halo spans a million light-years away from Andromeda. At this size, the halo could well be within halfway of our galaxy which is about 2.5 million light-years from its neighbor.

"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," said the lead investigator, Nicolas Lehner of the University of Notre Dame, Indiana.

Current understanding of galaxy halos is they are created by elements heavier than hydrogen and helium left after stars explode in supernovas. The explosions scatter away from the massive galactic center.

Researchers do not know if the Milky Way possesses a halo like Andromeda. If it does, it indicates the neighbors are merging well before their cores merge, to form a massive elliptical galaxy. The current timeline for a complete merger of the two galaxies into a giant elliptical galaxy is four billion years from now.