Reference: “Reconstructing the Last Major Merger of the Milky Way with the H3 Survey” by Rohan P. Naidu, Charlie Conroy, Ana Bonaca, Dennis Zaritsky, Rainer Weinberger, Yuan-Sen Ting, Nelson Caldwell, Sandro Tacchella, Jiwon Jesse Han, Joshua S. Speagle and Phillip A. Cargile, 14 December 2021, The Astrophysical Journal.DOI: 10.3847/ 1538-4357/ ac2d2d.
Presently one of our nearby neighbors, the Sagittarius dwarf galaxy, is being tidally disrupted (a dwarf galaxy has less than about 1% of the stellar mass of a typical spiral galaxy like the Milky Way, and often much less). Astronomers using the Gaia objective and the new H3 Survey of stars in the Milky Ways halo have shown that the Galaxys last major merger was with a dwarf system understood as Gaia-Sausage-Enceladus about 8-10 billion years earlier, and about half of the stars in the stellar halo descend from that system. CfA astronomers Rohan Naidu, Charlie Conroy, Ana Bonaca, Rainer Weinberger, Nelson Caldwell, Sandro Tacchella, Jiwon Han, and Phillip Cargile and their team used Gaia results integrated with a new survey of the external reaches of our Galaxy with the 6.5 m MMT telescope in AZ (the “H3 Survey”) to piece together the history of the Milky Ways stars in unprecedented information in order to figure out the nature of the Galaxys last merger. The Milky Way includes stars that are about 13 billion years old, although these may have been captured by the Galaxy after it formed.
Artists conception of the Milky Way galaxy. Credit: Pablo Carlos Budassi
Presently one of our nearby neighbors, the Sagittarius dwarf galaxy, is being tidally interfered with (a dwarf galaxy has less than about 1% of the outstanding mass of a regular spiral galaxy like the Milky Way, and often much less). The record of even more ancient mergers can be drawn out from the positions and movements of stars in the Milky Ways stellar halo, the approximately round circulation of stars (about one hundred thousand light-years in size) older than about 10-12 billion years. Andromeda, our nearest big neighboring galaxy, is about 10 times further away than these overshadows; a merger with it is anticipated in another five billion years.
A photograph of the Small Magellanic Cloud, a neighboring dwarf galaxy that is merging with the Milky Way. (The foreground globular cluster 47 Tucana is seen at the right.) Astronomers utilizing the Gaia objective and the new H3 Survey of stars in the Milky Ways halo have actually revealed that the Galaxys last major merger was with a dwarf system called Gaia-Sausage-Enceladus about 8-10 billion years earlier, and about half of the stars in the galactic halo descend from that system. Credit: Jose Mtanous
The Gaia spacecraft was released in 2013 with the objective of making an exact three-dimensional map of the Milky Way by surveying 1% of its roughly 100 billion stars. CfA astronomers Rohan Naidu, Charlie Conroy, Ana Bonaca, Rainer Weinberger, Nelson Caldwell, Sandro Tacchella, Jiwon Han, and Phillip Cargile and their group utilized Gaia results integrated with a brand-new survey of the external reaches of our Galaxy with the 6.5 m MMT telescope in AZ (the “H3 Survey”) to piece together the history of the Milky Ways stars in unmatched information in order to identify the nature of the Galaxys last merger. The evidence was already convincing that a single dwarf galaxy merged with the Milky Way about 8-10 billion years ago. Known as Gaia-Sausage-Enceladus (GSE), what is left of the things today is presumed from the stars in the inner halo by their excellent movements and structures. Still unsure, however, was whether GSE collided with our galaxy head-on, or if rather it orbited the galaxy before slowly combining, and if so, what that orbit appeared like.
They reveal that GSE contained about half a billion stars, and did not orbit the Milky Way but approached it moving in a retrograde direction (that is, opposite to the Galaxys rotational movement). The Milky Way includes stars that are about 13 billion years old, although these may have been recorded by the Galaxy after it formed.