Large galaxies like our Milky Way are the result of mergers of smaller galaxies. Although our galaxy survived, it has never been the same. With a good telescope, that dwarf galaxy's weird stars are still visible in our night sky; they've taken up residence in the solar neighbourhood and in the halo swaddling the Milky Way's spiral arms.
Put into orbit by the European Space Agency in 2013, Gaia has produced an unprecedented 3-D mapping of 1.7 billion stars, including more than a billion ― one per cent of the total ― in the Milky Way.
The team also studied the chemistry of almost 600 of those stars using ground-based telescope data, which confirmed that these stars had come from somewhere beyond the Milky Way. They discovered that about 30,000 of these stars were moving in the opposite direction of the other stars in the galaxy - a clear sign they may have originated elsewhere. What they observed with the Gaia data - and with data from the Apache Point Observatory's Galactic Evolution Experiment (APOGEE) in Chile - matched these simulations, leading them to conclude another galaxy had merged with the Milky Way.
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Large galaxies get that way by absorbing lesser ones. And it is one that relies on a single-yet massive-merger. "And they are a fairly homogeneous group, indicating that they share a common origin", she added. "That's when it became apparent that this was weird", she says. "The disk is ordered; you have 100 million stars moving orderly around the galactic center".
We are so deeply embedded in this collection that its stars surround us nearly completely, and so can be seen across most of the sky. "That hints that these stars could not have been formed in the Milky Way". In every galaxy the abundance of elements heavier than hydrogen and helium gradually increases over time due to cycles of stellar death and rebirth.
Artist's impression of the merger between the Gaia-Enceladus galaxy and our Milky Way, which took place during our Galaxy's early formation stages, 10 billion years ago.
The team named the galaxy that melded with the Milky Way Enceladus ― progeny of Gaia, goddess of Earth, and Uranus, god of the sky ― because the giant was said to have caused earthquakes after being buried under Mount Etna, much in the way the rogue galaxy unsettled and resculpted the Milky Way.
The study, if correct, firstly confirms what theorists have long thought: that galaxies like the Milky Way grow to enormous proportions by devouring many smaller ones. "It wasn't known whether the Milky Way had experienced any mergers", Helmi says. "And you never know when you see a merger in another galaxy, if it's just anecdotal".
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"It's very cool that stars that formed in another galaxy could be lurking right next door to us", said Kim Venn, a professor at the University of Victoria's department of physics and astronomy, who was not involved in the study.
The Milky Way has a number of distinct structures: The central bulge, spiral arms, the disk and the halo. But how many and when was these clashes is unknown.
Gaia-Enceladus isn't the only galaxy the Milky Way has gobbled up, Columbia University Astronomer Kathryn Johnston tells Meghan Bartels at Space.com.
And the answer will only further shed light on these galactic wrecks.
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