Far out cosmology: the Milky Way is unusual (Introduction)

by David Turell , Thursday, November 21, 2024, 14:33 (6 hours, 19 minutes ago) @ David Turell

Latest study on satellite galaxies:

"...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."

"To achieve that goal, Wechsler cofounded the Satellites Around Galactic Analogs (SAGA) Survey dedicated to comparing galaxies similar in mass to the Milky Way.

"After more than a decade of scanning the universe, the SAGA team identified and studied 101 Milky Way-like analogs as a first step in its ongoing research. The results, published in three studies in the Nov. 18 issue of The Astrophysical Journal, reveal that, in many ways, the evolutionary history of the Milky Way is different from other comparable-sized galaxies.

"'Our results show that we cannot constrain models of galaxy formation just to the Milky Way," said Wechsler, who is also 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."

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"Studies show that galaxies form inside 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 pull in ordinary matter from space and transform it into stars and galaxies.

"A key objective of the SAGA Survey is to determine how dark matter halos impact galactic evolution. To begin, the SAGA team focused on galactic satellites—small galaxies that orbit much larger host galaxies like the Milky Way.

"The researchers identified four of the Milky Way's brightest satellite galaxies, including the two biggest, known as the Large and Small Magellanic Clouds (LMC and SMC). The scientists then conducted a painstaking search for satellites around other host galaxies similar in mass. Using telescopic imaging, they eventually identified 378 satellite galaxies surrounding 101 Milky Way-like hosts.

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"In one of the three new SAGA studies, researchers found that the number of satellites per host galaxy ranges from 0 to 13. The Milky Way's four observable satellites fit within 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, smaller satellites are still forming stars. But in the Milky Way, star formation only occurs in the massive LMC and SMC satellites. 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 have their star formation quenched? Perhaps, unlike a typical host galaxy, the Milky Way has a unique combination of older satellites that have ceased star formation and newer, active ones—the LMC and SMC—that only recently fell into the Milky Way's dark matter halo."

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

"'To me, the frontier is figuring out what dark matter is doing on scales smaller than the Milky Way, like with the smaller dark matter halos that surround these little satellites," Wechsler said.

"The third study, led by Stanford doctoral scholar Yunchong "Richie" Wang, compares the new data to computer simulations and calls for the development of a new model of galaxy formation based in part on the SAGA Survey."

Comment: as usual the Milky Way stands out as unusual. From the designer standpoint the Milky Way must protect the Earth and that explains the differences.