Puffy Milky Way

Is our current model of the universe correct?

Density maps of disk stars illustrating the global morphological transformation of a galactic disk subject to bombardment by dark matter substructures. Brighter colors indicate regions of higher density of disk stars. The left panel shows the initial disk, while the right panel depicts the final disk after the violent gravitational encounters with the orbiting substructures.

Density maps of disk stars illustrating the global morphological transformation of a galactic disk subject to bombardment by dark matter substructures. Brighter colors indicate regions of higher density of disk stars. The left panel shows the initial disk, while the right panel depicts the final disk after the violent gravitational encounters with the orbiting substructures.

Their simulations offer a new way to test and validate the current cosmological model of the universe.

According to the model, the universe has contained a certain amount of normal matter and a much larger amount of dark matter, starting with the Big Bang. The exact nature of dark matter is unknown, and scientists are hunting for clues by studying the interplay between dark matter and normal matter.

This is the first time that collisions between spiral galaxies and satellites have been simulated at this level of detail, Kazantzidis said, and the study revealed that galaxies’ flared edges and stellar rings are visible signs of these interactions.

Our galaxy measures 100,000 light-years across (one light year equals six trillion miles). Yet we are surrounded by a cloud or “halo” of dark matter that’s 10 times bigger -- 1 million light-years across, he explained.

While astronomers envision the dark matter halo as partly diffuse, it contains dense regions that orbit our galaxy in association with satellite galaxies, such as the Magellanic Clouds.

“We know from cosmological simulations of galaxy formation that these smaller galaxies probably interact with galactic disks very frequently throughout cosmic history. Since we live in a disk galaxy, it is an important question whether these interactions could destroy the disk,” Kazantzidis said. “We saw that galaxies are not destroyed, but the encounters leave behind a wealth of signatures that are consistent with the current cosmological model, and consistent with our observations of galaxies in the universe.”

One signature is the flaring of the galaxy’s edges, just as the edges of the Milky Way and of other external galaxies are flared.

We consider this flaring to be one of the most important observable consequences of interactions between in-falling satellite galaxies and the galactic disk.”

In both articles, the researchers considered the impacts of many different smaller galaxies onto a larger, primary disk galaxy. They calculated the likely number of satellites and the orbital paths of those satellites, and then simulated what would happen during collision, including when the dark matter interacted gravitationally with the disk of the spiral galaxy.

None of the disk galaxies were torn apart; to the contrary, the primary galaxies gradually disintegrated the in-falling satellites, whose material ultimately became part of the larger galaxy.

The satellites passed through the galactic disk over and over, and on each pass, they would lose some of their mass, a process that would eventually destroy them completely.

Though the primary galaxy survived, it did form flared edges which closely resembled our galaxy’s flared appearance today.

“Every spiral galaxy has a complex formation and evolutionary history,” Kazantzidis said. “We would hope to understand exactly how the Milky Way formed and how it will evolve. We may never succeed in knowing its exact history, but we can try to learn as much as we can about it, and other galaxies like it.”

Ohio State University
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