Satellite Galaxies Show Milky Way May Not Be A Typical Spiral Galaxy

Our home galaxy, the Milky Way, is the one that has been most thoroughly studied by astronomers, given its proximity. And while much is still unknown about our spiral galactic home — on whose outer edge we live — our knowledge of the Milky Way and its surroundings is used by scientists to create models for understanding the rest of the universe, most of which is much farther away.

However, these models are likely to work as expected only if the Milky Way is a typical, run-of-the-mill type of galaxy that behaves in much the same way as most other galaxies in the known universe. Early results from an ongoing space survey point to the contrary, suggesting the Milky Way does not display behavior that is observed in comparable galaxies.

Along with hosting Earth, the solar system and billions of stars, the Milky Way is also the center of many other, smaller satellite galaxies that orbit around it. Studying these satellite galaxies helps in understanding the Milky Way as well. And that is where the discrepancy arises.

Satellite galaxies of other large galaxies that are of comparable luminosity and environment to the Milky Way — referred to as “siblings” — are seen to be actively producing new stars, while the galaxies that orbit the Milky Way are relatively “tranquil” and “mostly inert,” according to findings of the Satellites Around Galactic Analogs (SAGA) Survey.

“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,” Marla Geha, an astrophysicist at Yale University and lead author of a paper on the subject, said in a statement Wednesday.

“The SAGA Survey aims to determine dwarf galaxy satellite systems around 100 Milky Way analogs,” according to its website. A total of 16 satellite galaxies have been discovered by it so far from eight sibling systems, bringing the total number of known satellite galaxies from such systems to 29, the remaining 13 having been discovered by the Sloan Digital Sky Survey.

Given that only eight sibling systems have been studied by the SAGA Survey so far, its findings cannot be used to arrive at any firm conclusions. Over the course of the next two years, another 17 sibling systems are expected to be studied by the survey.

“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.

Risa Wechsler, an astrophysicist at the Kavli Institute at Stanford University, who is a SAGA researcher and was a co-author on the paper, said in the statement: “Our work puts the Milky Way into a broader context. The SAGA Survey will provide a critical new understanding of galaxy formation and of the nature of dark matter.”

The paper, titled “The SAGA Survey. I. Satellite Galaxy Populations around Eight Milky Way Analogs,” appeared online last week in the Astrophysical Journal.