The Most Recent Major Collision in Our Milky Way Galaxy.

Artist’s conception of the Milky Way galaxy. Credit: Pablo Carlos Budassi
Artist’s conception of the Milky Way galaxy. (Image Credit: Pablo Carlos Budassi)

One of the distinguishing elements of current cosmology is its account of how galaxies evolve: a hierarchical process of colliding and merging with other systems. Nowhere in the cosmos do we get a better perspective of this accumulation than in our Milky Way. The Sagittarius dwarf galaxy, one of our close neighbours, is now experiencing tidal disruption (a dwarf galaxy has less than about 1 per cent of the stellar mass of a typical spiral galaxy like the Milky Way, and often much less). Dwarf stars in the Large and Small Magellanic Clouds are also heading our way. Their stellar masses are just approximately one per cent and one-hundredth of the Milky Way's, respectively.
Meanwhile, streams of globular clusters ring the Galaxy, showing the consequences of earlier mergers. The record of even more ancient unions may be derived from the locations and movements of stars in the Milky Way's stellar halo, the approximately spherical dispersion of stars (about one hundred thousand light-years in diameter) older than around 10-12 billion years. Meanwhile, Andromeda, our closest major neighbouring Galaxy, is nearly 10 times further away than these dwarfs; a merger with it is projected in about five billion years.

A photograph of the Small Magellanic Cloud, a nearby dwarf galaxy that is merging with the Milky Way. (The foreground globular cluster 47 Tucana is seen at the right.) Astronomers using the Gaia mission and the new H3 Survey of stars in the Milky Way’s halo have shown that the Galaxy’s last major merger was with a dwarf system known as Gaia-Sausage-Enceladus about 8-10 billion years ago, and about half of the stars in the galactic halo descend from that system. (Credit: Jose Mtanous)

Gaia was launched in 2013 to scan 1% of the Milky Way's estimated 100 billion stars to create an accurate three-dimensional map. For the "H3 Survey," a new survey of the outer reaches of our Galaxy with the 6.5m MMT telescope in Arizona (the "H3 Results") and data from the Gaia mission were used to piece together the history of the Milky Way's stars in unprecedented detail, allowing researchers to determine the nature of the Galaxy's most recent merger. The team included CfA astronomers Rohan Naidu, Charlie Conroy, Ana Bonaca, Rainer Weinberger, Nelson Caldwell. The data suggested that the Milky Way and dwarf galaxies merged some 8-10 billion years ago. Known as Gaia-Sausage-Enceladus (GSE), what is left of the object today is deduced from the stars in the inner halo by their stellar movements and compositions. Still unknown, though, was whether GSE smashed with our galaxy head-on, or if instead, it orbited the Galaxy before gradually merging, and if so, what that circled looked like.

The astronomers used numerical simulations and comparisons with stellar ages and compositions to simulate Gaia's observed halo stars and answer these issues. According to their findings, GSE comprised around half a billion stars and did not revolve around the Milky Way but instead approached it travelling backwards. They also find that approximately 50 per cent of the Milky Way's present star halo and about 20 per cent of its dark matter halo descend from it. The Milky Way includes stars that are roughly 13 billion years old. However, they may have been caught by the Galaxy after it formed. With the conclusion of this research, however, practically the whole expansion of the Milky Way over the previous 10 billion years can be accounted for.


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