A recent study went full Indiana Jones to show that our home galaxy, the Milky Way, was a cannibal in its earlier years, swallowing five smaller galaxies.
Like Jones, the famed (albeit fictional) archaeologist, astrophysicist Duncan Forbes of Swinburne University in Australia used clues from past events to reconstruct our cosmic neighborhood's history. In this case, he used globular clusters — groups of ancient stars — orbiting the Milky Way, to show our galaxy's cannibalistic history.
Forbes' recent research (for which he was the sole author) examined properties such as the clusters' age, what they are made of, their spin (or angular momentum) and how much energy they expend in their orbits. The age and chemistry of these stars have long been known, but their spin and expended energy were only narrowed down more recently thanks to observations from the European Gaia satellite, which tracks the positions and luminosity of a billion stars.
These clues, when put together in a model, show a picture of which clusters originally "belonged" to the Milky Way, and where any outliers originally came from. While the universe looks like a static thing when gazing at the night sky, astronomers are able to see large shifts and changes over millions and billions of years.
Galaxies may collide or swallow smaller galaxies whole, and Forbes saw ample evidence of this cosmic cannibalism happening in our galaxy.
"I was able to identify individual globular clusters that did not form in the Milky Way, but rather formed in dwarf galaxies that were later acquired by the Milky Way," Forbes told Space.com in an e-mail.
Roughly half of the 200 globular clusters studied in this research were accreted (or absorbed) into the Milky Way over time. Forbes has several colorful names for these clusters, including Koala — a moniker for the "low-energy" group, since koalas sleep about 18 hours a day. (In this study, "low energy" refers to orbits that are more tightly bound to the Milky Way, and less likely to escape our galaxy's pull.)
Forbes concluded the globular clusters came from five satellite galaxies swallowed up long ago, between 5 billion and 11 billion years before the present day. (The universe, for comparison, is roughly 13.8 billion years old.) Individual accretion events would have taken a few million years each, which is long compared to a human's lifespan, but short compared with the history of the universe.
While Forbes was coy about what he plans to study next, he hinted that Andromeda could be an intriguing target for researchers looking to take a similar approach. "It has around 500 globular clusters, and so would be interesting to contrast with the Milky Way's assembly history," he said.
A study based on the research was published in late January in the Monthly Notices of the Royal Astronomical Society.
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Elizabeth Howell (she/her), Ph.D., is a staff writer in the spaceflight channel since 2022 covering diversity, education and gaming as well. She was contributing writer for Space.com for 10 years before joining full-time. Elizabeth's reporting includes multiple exclusives with the White House and Office of the Vice-President of the United States, an exclusive conversation with aspiring space tourist (and NSYNC bassist) Lance Bass, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, flying parabolic, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, "Why Am I Taller?", is co-written with astronaut Dave Williams. Elizabeth holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, a Bachelor of Journalism from Canada's Carleton University and a Bachelor of History from Canada's Athabasca University. Elizabeth is also a post-secondary instructor in communications and science at several institutions since 2015; her experience includes developing and teaching an astronomy course at Canada's Algonquin College (with Indigenous content as well) to more than 1,000 students since 2020. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday. Mastodon: https://qoto.org/@howellspace
"While the universe looks like a static thing when gazing at the night sky, astronomers are able to see large shifts and changes over millions and billions of years."Reply
Jeez, did not know that astronomers lived so long! I got into the wrong field of science!
Seriously, this is interesting work. What is not clear is how half the globular clusters (GCs) could have been derived from only 5 galaxies. Since they were absorbed in the distant past, it seems likely that each of these assimilated galaxies were themselves amalgamations with many GCs. Otherwise, how do you get so many GCs from only 5 galaxies? Since the Milky Way has so many native GCs (nGCs), based on this article, it stands to reason that assimilated galaxies did also.
The data on spin of the clusters is very interesting. One wonders how their spin directions compare with each other. Do all or most of the spinning GCs have "parallel" spins, or do they all spin in highly variable "directions"? Perhaps there are groups that spin with similar direction.
One assumes that the nGCs likely formed around localized high density hydrogen clouds in the early Milky Way, or all are actually acquired , and Forbes just hasn't figured this out. I suspect these nGCs lack spin. Another issue would be the amount of gas in these GCs. Acquired GCs may be more gas poor than nGCs, since the latter should have being drawing gas from the galaxy throughout its life, but the captured ones cannot for some reason. I suspect a lot may depend on their movement through the galaxy (in and out of the plane, or stable).
Finally, if the Milky Way absorbed all those galaxies with all those clusters, I suspect it too was a minor galaxy, and the addition of 5 other galaxies put the combination into the class of giant spiral galaxies. There is clearly more to this story than the Milky Way eating 5 galaxies.