Milky Way grew by 'cannibalising' other smaller galaxies

Astronomers using NASA's Hubble Space Telescope have uncovered tantalising evidence for the possible existence of a shell of stars that are a relic of the Milky Way's past cannibalism of other galaxies.
Peering deep into the vast stellar halo that envelops our Milky Way galaxy, a team of astronomers led by Alis Deason, a from UC Santa Cruz, used Hubble observations to precisely measure, for the first time ever, the sideways motions of a small sample of stars located far from the galaxy's center.
Their unusual lateral motion is circumstantial evidence that the stars may be the remnants of a shredded galaxy that was gravitationally ripped apart by the Milky Way billions of years ago. These stars support the idea that the Milky Way grew through the accretion of smaller galaxies.

"Hubble's unique capabilities are allowing astronomers to uncover clues to the galaxy's remote past," said coauthor Roeland van der Marel of the Space Telescope Science Institute (STScI) in Baltimore.
"The more distant regions of the galaxy have evolved more slowly than the inner sections. Objects in the outer regions still bear the signatures of events that happened long ago," Marel said in a statement.
They also offer a new opportunity for measuring the "hidden" mass of our galaxy, which is in the form of dark matter - an invisible form of matter that does not emit or reflect radiation.

"Our ability now to measure the motions of these stars opens up a whole new territory we haven't explored yet," Deason said.

Deason and her team plucked the outer halo stars out of seven years' worth of archival Hubble telescope observations of our neighbouring Andromeda galaxy.
In those observations, Hubble peered through the Milky Way's halo to study the Andromeda stars, which are more than 20 times farther away.
The Milky Way's halo stars were in the foreground and considered as clutter for the study of Andromeda. But to Deason's study they were pure gold.
The observations offered a unique opportunity to look at the motion of Milky Way halo stars.
"We knew these stars were there, because for the Andromeda study we had to separate the stars in Andromeda from the stars in the Milky Way," said coauthor Puragra Guhathakurta, professor of astronomy and astrophysics at UC Santa Cruz.

Researchers said finding the stars was meticulous work. Each Hubble image contained more than 100,000 stars.
"We had to somehow find those few stars that actually belonged to the Milky Way halo. It was like finding needles in a haystack," Marel said.