The Fate of the Milky Way
As scientists attempt to learn more about how galaxies evolve, an open question has been whether collisions with our dwarf galactic neighbors will one day tear apart the disk of the Milky Way.
Kazantzidis couldn’t offer a 100-percent guarantee, however.
“We can’t know for sure what’s going to happen to the Milky Way, but we can say that our findings apply to a broad class of galaxies similar to our own,” Kazantzidis said. “Our simulations showed that the satellite galaxy impacts don't destroy spiral galaxies -- they actually drive their evolution, by producing this flared shape and creating stellar rings -- spectacular rings of stars that we’ve seen in many spiral galaxies in the universe.”
He and his colleagues didn’t set out solely to determine the fate of our galaxy. In two papers that have appeared in the Astrophysical Journal, they report that 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.
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.
“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.”
His coauthors included James Bullock from the University of California at Irvine, Andrew Zentner from the University of Pittsburgh, Andrey Kravtsov from the University of Chicago, Leonidas Moustakas from NASA’s Jet Propulsion Laboratory (JPL) , and Victor Debattista from the University of Central Lancashire in the UK.
Kazantzidis’ research was funded by the Center for Cosmology and Astro-Particle Physics at Ohio State. Other funding came from the National Science Foundation, NASA, the University of Pittsburgh, and the University of Chicago. The numerical simulations were performed on the zBox supercomputer at the University of Zurich and on the Cosmos cluster at JPL.