Is Our Galaxy Unique? A Decade-Long Study Sheds New Light on Our Place in the Universe

After surveying almost 50,000 galaxies in a study that took just over 10 years, astronomers have identified a relatively small number of star systems with important similarities to our own. The discovery could provide scientists with important information about how those systems are created, evolve and die and where our own galaxy is on that journey.

“This study was an attempt to really contextualize the Milky Way. Are we just like all these other galaxies, or are we somehow special?” said Nitya Kallivayalil, a professor of astronomy with UVA’s College and Graduate School of Arts & Sciences and co-author of a series of papers published by astronomers affiliated with the Satellites Around Galactic Analogs (SAGA) Survey team, a project launched in 2012 to identify galaxies similar to the Milky Way.

Using high-resolution data collected by telescopes in Arizona and Australia, the SAGA astronomers were able to identify 101 galaxies that are similar in size to our own and several hundred smaller galaxies orbiting around them. The data will allow astronomers to assess whether the satellite populations of these Milky Way analogs are similar to our own.

Studies like the SAGA survey are the first attempt to get a similar amount of detail for other galaxies and are an important tool for testing theories about the universe.

“In the Milky Way we can make really detailed high-resolution observations, and we can see individual stars,” Kallivayalil said. “It’s really hard to hide things in the data because it’s such high resolution.” 

New Insight into Our Own Place in the Universe

So how unique is our galaxy?

“It turns out that the Milky Way is pretty typical, and it’s atypical,” Kallivayalil said. “It’s good that it’s typical, because that means we can continue to study it in this way, in exquisite, detail and continue to get some broad takeaways. But the ways it’s atypical are pretty intriguing as well, and those anomalies are going to teach us a lot.”

Dynamical history is hard to get in astronomy because you’re essentially relying on snapshots, she explained. 

“But this has enabled us to collect velocity information about similar galaxies, which allows you to back out a past and a future trajectory,” said Kallivayalil, whose work with the SAGA Survey will provide researchers and students with the raw materials for discovery and new knowledge about the universe for years to come.

“Professor Kallivayalil’s pioneering work is nothing short of transformative,” said Christa Acampora, Buckner W. Clay Professor of Philosophy and Dean of Arts & Sciences. “Projects like the SAGA Survey push the boundaries of human knowledge, challenging us to rethink our place in the cosmos. They inspire this generation and the next to confront the fundamental questions that shape our existence. We are truly fortunate to have this talent, vision and expertise right here at the University of Virginia.”

Data from the survey could also hold the key to another of the universe’s mysteries, one that could help astronomers understand what’s happening in the Milky Way.

Dark matter is essentially the skeleton on which galaxies grow, providing the gravity that brings normal matter in and allowing proto-galaxies and stars to form. However, distinguishing between the effects of normal physical processes and the effects caused by dark matter is difficult, according to Kallivayalil. That holds especially true with regard to its effects on the birth of stars and their deaths and the role of supermassive black holes, all of which add to the energy and momentum of galaxies.

Our galaxy has a smaller number of satellites than a typical Milky Way type galaxy, and there are no new stars forming in them, Kallivayalil said. 

“What we think this means is that it’s related to the specific history of the Milky Way and how it formed. “We think what’s going on in the Milky Way is that it formed relatively early in the history of the universe. It’s a slightly older, slightly less massive system.”

According to Kallivayalil, the data set will play a critical role in helping astronomers and physicists develop a systematic approach to understanding not just our own galaxy but it will also offer insight into the nature of dark matter and the role it plays in the universe.

“To me, there is no more interesting question than what is the universe made of, and is it made of the same things we’re made of,” Kallivayalil said. “The stuff we’re made of — the protons and neutrons of normal matter — is only 5% of the universe, so there’s 95% of the universe that’s unknown to us. To me, that boggles the mind. We owe our existence to dark matter, and I feel like I need to figure out what it is.”