The Milky Way represents an atypical case among similar galaxies

Nov. 20 () –

Three new studies co-led by Stanford scientists reveal that the birth of our galaxy may not be typical of how other galaxies evolved.

For decades, scientists have used the Milky Way as a model to understand how galaxies form. But a trio of new studies raise questions about whether the Milky Way is really representative of other galaxies in the universe.

“The Milky Way has been an incredible physics laboratory, including for the physics of galaxy formation and the physics of dark matter,” he said. in a statement Risa Wechsler, professor of Humanities and Sciences and professor of physics in the School of Humanities and Sciences. “But the Milky Way is just one system and may not be typical of how other galaxies formed. That’s why it’s critical to find similar galaxies and compare them.”

To achieve that goal, Wechsler co-founded the Satellites Around Galactic Analogs (SAGA) project, dedicated to comparing galaxies similar in mass to the Milky Way. After more than a decade of exploring the universe, the SAGA team identified and studied 101 analog galaxies similar to the Milky Way as a first step in their ongoing research. The results, published in three studies in the November 18 issue of The Astrophysical Journal, reveal that, in many ways, The evolutionary history of the Milky Way is different from that of other galaxies of comparable size.

“Our results show that we cannot limit galaxy formation models to just the Milky Way,” said Wechsler, who is also a professor of particle physics and astrophysics at SLAC National Accelerator Laboratory. “We have to look at that full distribution of similar galaxies throughout the universe.”

In addition to Wechsler, the SAGA project is led by Professor Marla Geha of Yale University and Yao-Yuan Mao, a former doctoral student of Wechsler’s at Stanford, now an assistant professor at the University of Utah. All three are co-authors of the recently published studies.

The Milky Way is made up of ordinary atomic matter, such as hydrogen and iron. But ordinary matter only represents about 15% of the matter in the universe. The remaining 85% is mysterious and invisible dark matter.

“No one knows what dark matter is made of,” Wechsler said. “It doesn’t interact with ordinary matter or light. There’s probably dark matter circulating through you right now and you don’t even know it.”

Studies show that galaxies form within massive regions of dark matter called halos. A dark matter halo may be invisible, but its enormous size creates a gravitational force strong enough to attract ordinary matter from space and transform it into stars and galaxies.

A key goal of the SAGA survey is to determine how dark matter halos affect galactic evolution. To start, the SAGA team focused on galactic satellites, small galaxies that orbit much larger host galaxies, such as the Milky Way. Researchers identified four of the Milky Way’s brightest satellite galaxies, including the two largest, known as the Large and Small Magellanic Clouds (LMC and SMC). The scientists then carried out a thorough search for satellites around other host galaxies of similar mass. Using telescopic images, they eventually identified 378 satellite galaxies surrounding 101 Milky Way-like hosts.

“There’s a reason no one has tried this before,” Wechsler said. “It’s a really ambitious project. We had to use clever techniques to separate those 378 orbiting galaxies from thousands of objects in the background. It’s a real needle-in-a-haystack problem.”

THREE STUDIES

In one of three new SAGA studies, researchers found that the number of satellites per host galaxy ranges from zero to 13. The Milky Way’s four observable satellites fit into that range.

The study also revealed that host galaxies with large satellites, similar in size to the Milky Way’s massive LMC and SMC galaxies, tend to have more satellites overall. But the Milky Way actually hosts fewer satellites than similar galaxies, making it an outlier among its peers..

A second study focused on star formation in satellite galaxies, an important metric for understanding how galaxies evolve. The study found that in a typical host galaxy, the smallest satellites are still forming stars. But in the Milky Way, star formation only occurs in the massive satellites LMC and SMC. All the smaller satellites have stopped forming stars.

“Now we have a puzzle,” Wechsler said. “What in the Milky Way caused these small, lower-mass satellites to see their star formation extinguished? Perhaps unlike a typical host galaxy, the Milky Way has a unique combination of older satellites that have stopped forming stars and newer, more active galaxies (the LMC and the SMC) that recently fell into the Milky Way’s dark matter halo.”

The study also found that star formation generally stops in satellite galaxies located closer to the host galaxy, perhaps due to the gravitational pull of dark matter halos in and around the host galaxy.

“For me, the frontier is figuring out what dark matter does on scales smaller than the Milky Way, as in the smaller dark matter halos that surround these small satellites,” Wechsler said.

The third study, led by Stanford doctoral researcher Yunchong “Richie” Wang, compares the new data with computer simulations and calls for the development of a new model of galaxy formation based in part on the SAGA survey.

“SAGA provides a benchmark to advance our understanding of the universe through the detailed study of satellite galaxies in systems beyond the Milky Way,” Wechsler said. “Although we finished our initial goal of mapping bright satellites in 101 host galaxies, there is much more work to do.”