Our galactic home, the spiral-armed Milky Way, might not behave like similar galaxies, according to findings of the Satellites Around Galactic Analogs (SAGA) Survey. The findings suggest current models for both galactic and stellar formation might require revising.

“We use the Milky Way and its surroundings to study absolutely everything. 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,” said Marla Geha, an astrophysicist at Yale University and lead author of the new paper on the subject.

One of the Milky Way’s sibling as seen by the Sloan Digital Sky Survey.

Just like moon orbits Earth or Earth orbits the sun, so do other smaller galaxies revolve around the Milky Way’s core, such as the Large and Small Magellanic Clouds. Andromeda, a neighboring spiral galaxy, also has similar satellites. In turn, the Milky Way revolves around the Local Group center of mass, located somewhere between Andromeda and the Milky Way. Nothing is stationary and everything travels with respect to something else.

The movements and behaviors of galactic satellites can indirectly offer information about their galactic parent. In 2012, an international collaboration launched the SAGA Survey with the stated goal of studying satellite galaxies around a hundred or so spiral galaxies similar to the Milky Way. So far, five such galaxies and their satellites have been tracked. To reach definite conclusions, at least 25 galaxies should be studied, a target which ought to be reached within two years.

Already, however, the survey is reporting conflicting findings with what we’d expect. According to Geha, the satellites of the Milky Way’s siblings behave significantly different. One important distinction is our galactic satellites are mostly ‘inert’ in terms of new star formation while those of the Milky Way’s siblings are far more active, pumping out new stars at a high rate.

As such, the work puts the Milky Way in a far grander context. By one estimate, there are one hundred billion galaxies in the observable universe but we’re only beginning how similar or, by contrast, dissimilar these galaxies can be.

“I really want to know the answer to whether the Milky Way is unique, or totally normal. By studying our siblings, we learn more about ourselves,” Geha said.

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