Our galaxy belongs to the class of spiral galaxies. Scientists have studied 101 analogs of the Milky Way and counted their number of satellites. However, it turned out that our star system is atypical in this respect.
The Milky Way as a model for study
For decades, scientists have used the Milky Way as a model for understanding how galaxies form. But three new studies raise the question of 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,” said Risa Wechsler, professor of humanities and sciences and professor of physics in the School of Humanities and Sciences. “But the Milky Way is only 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.”
Detecting satellites of galaxies
To accomplish this goal, Wechsler co-founded the Satellites Around Galactic Analogs (SAGA) study to compare galaxies similar in mass to the Milky Way.
After more than a decade of scanning the Universe, the SAGA team has discovered and studied 101 Milky Way analog galaxies, the first step in its ongoing research. The results, published in three studies in the November 18 issue of The Astrophysical Journal, show that the evolutionary history of the Milky Way differs in many ways from other galaxies of comparable size.
“Our results show that we cannot constrain models of galaxy formation just to the Milky Way,” said Wechsler, who is also a professor of particle physics and astrophysics at the SLAC National Accelerator Laboratory. “We have to look at that full distribution of similar galaxies across the universe.”
Mystery of dark matter
The Milky Way is made up of ordinary atomic matter such as hydrogen and iron. But ordinary matter accounts for only about 15% of the matter in the Universe. The other 85% is mysterious, invisible dark matter.
Research shows that galaxies form inside massive regions of dark matter called halos. The dark matter halo may be invisible, but its enormous size creates a gravitational force strong enough to attract ordinary matter from space and turn it into stars and galaxies.
A key goal of the SAGA survey is to determine how dark matter halos affect galactic evolution. First, the SAGA team focused on galactic satellites — small galaxies that orbit around much larger host galaxies such as the Milky Way.
Researchers have identified the four brightest satellite galaxies in the Milky Way, including the two largest, known as the Large and Small Magellanic Clouds (LMO and SMO). Next, scientists conducted a meticulous search for satellites around other host galaxies similar in mass. Using telescopic images, they eventually found 378 satellite galaxies around 101 Milky Way-like galaxies.
“There’s a reason no one ever tried this before,” Wechsler said. “It’s a really ambitious project. We had to use clever techniques to sort those 378 orbiting galaxies from thousands of objects in the background. It’s a real needle-in-the-haystack problem.”
New results
In one of three new SAGA studies, scientists found that the number of satellites per host galaxy ranges from 0 to 13. Four observed Milky Way satellites fit within this range.
The study also found that host galaxies with large satellites, similar in size to the Milky Way’s massive LMC and SMC, tend to have more satellites. But the Milky Way actually has fewer satellites than similar galaxies, making it special among other galaxies.
The second study focused on star formation in satellite galaxies, an important metric for understanding how galaxies evolve. The study found that in a typical parent galaxy, smaller companions still form stars. But star formation in the Milky Way occurs only in the massive LMC and SMC satellites. All smaller satellites have ceased to form stars.
Provided by phys.org
