When you think of the Milky Way, you probably think of home, but what makes a galaxy home doesn’t necessarily make it typical.
Galaxies have satellites just as planets have moons. Through the SAGA (Satellites Around Galactic Analogs) survey and other studies, the smaller galaxies that orbit our own tell scientists things about the Milky Way that might not have been so obvious without them. SAGA’s mission is to examine the satellites floating around a hundred galaxies similar to our own, and our galaxy’s satellites are something like the equivalent of cosmic couch potatoes. They are surprisingly inert when compared with others that have similar environments and luminosities. That is, they don’t constantly create more stars.
This is kind of a big deal, because many ways in which we understand the universe use the Milky Way as a model for how galaxies behave. Except we may have gotten it all backward.
"We use the Milky Way and its surroundings to study absolutely everything," said astrophysicist Marla Geha, whose research team recently published a study in the Astrophysical Journal. "Hundreds of studies come out every year about dark matter, cosmology, star formation, and galaxy formation, using the Milky Way as a guide. But it's possible that the Milky Way is an outlier."
SAGA is still far from going through those hundred satellites, having only examined eight (which really adds up to 27 when you take the 13 known satellites into account), with 25 expected within the next two years. Even at this early phase, the absence of new stars in most of the Milky Way’s satellites has been opening more than a few eyes.
"Our work puts the Milky Way into a broader context," said astrophysicist and SAGA researcher Risa Wechsler. "The SAGA Survey will provide a critical new understanding of galaxy formation and the nature of dark matter.”
Yes, she just said dark matter. Think about it. There might be something in the Milky Way’s distant past that could explain why more stars aren’t shining. If our galaxy is anomalous, would it really be an optimal model for demystifying galactic formation and dark matter?
The SAGA team will further illuminate whether the lack of star formation really is that strange as they continue to devise more efficient methods of finding satellites around sister galaxies. Putting our galaxy into a cosmological context by comparing it to analogous systems will give us a better sense of how we fit in—or stand out.