The Andromeda Galaxy’s halo is more massive than initially thought

Galactic halos are both more massive and more complicated than scientists realised, according to new Hubble observations. The venerable telescope turned its sights on the neighbouring Andromeda Galaxy using dozens of different quasars to map the galactic halo. Andromeda, more formally known as Messier 31, is a spiral-shaped galaxy about the same size of the Milky Way we live in, with about

1 trillion stars. Cosmically it’s right next door, just 2.5 million light years away, which means that Hubble can study its halo in unprecedented detail.

“This is truly a unique experiment because only with Andromeda do we have information on its halo along not only one or two sightlines, but over 40,” said Nicolas Lehner, an astrophysicist at the University of Notre Dame in Indiana. “This is groundbreaking for capturing the complexity of a galaxy halo beyond our own Milky Way.”

Researchers found that the halo itself stretched much farther across space than expected, a whopping 1.3 million light years out from the galaxy, and at some spots more like 2 million light years. If human eyes could see it, it would be three times as wide as the Big Dipper. In starker terms, Andromeda’s halo covers more than half the distance between Andromeda and the Milky Way, suggesting that the two halos intermingle.

The research also found that the structure of Andromeda’s halo is more complicated than expected, with two distinct layers. “We find the inner shell, that extends to about half a million light years, is far more complex and dynamic,” Lehner said. “The outer shell is smoother and hotter.”