“We have now simulated, for the first time, the RAR of dwarf galaxies on the assumption that dark matter exists,” explains Prof. Dr. Cristiano Porciani of the Argelander Institute for Astronomy at the University of Bonn. “It turned out that they behave as scaled-down versions of larger galaxies.” But what if there is no dark matter and instead gravity works differently than Newton thought? “In this case, the RAR of dwarf galaxies depends strongly on the distance to their parent galaxy, while this does not happen if dark matter exists,” explains researcher Emilio Romano-Díaz.
Researchers at the University of Bonn and the University of California at Irvine used sophisticated computer simulations to devise a test that could answer a burning question in astrophysics: does dark matter actually exist? Or does Newton’s gravitational law need to be modified? The new study, now published in the Physical Review Letters, shows that the answer is hidden in the motion of the stars within small satellite galaxies swirling around the Milky Way.
Using one of the fastest supercomputers in the world, the scientists have simulated the matter distribution of the so-called satellite “dwarf” galaxies. These are small galaxies that orbit larger galaxies like the Milky Way or Andromeda with its 14 orbiting dwarf satellites.
“When we looked at the dwarf galaxies surrounding Andromeda, we expected to find them buzzing around randomly, like angry bees around a hive,” said Geraint F. Lewis at the Sydney Institute for Astronomy at the University of Sydney. “Instead, we’ve found that half of Andromeda’s satellites are orbiting together in an immense plane, which is more than a million light years in diameter but only 30 000 light years thick. These dwarf galaxies have formed a ring around Andromeda. This was completely unexpected – the chance of this happening randomly is next to nothing. It really is just weird.”
Large galaxies, like Andromeda and our own Milky Way, have long been known to be orbited by an entourage of smaller galaxies. These small galaxies, which are individually anywhere from ten to at least hundreds of thousands of times fainter than their bright hosts, were thought to trace a path around the big galaxy that was independent of every other dwarf galaxy. For several decades, astronomers have used computer models to predict how dwarf galaxies should orbit large galaxies, and every time they found that dwarfs should be scattered randomly over the sky. Never, in these synthetic universes, did they see dwarfs arranged in a plane like that observed around Andromeda.
“Now that we’ve found that the majority of these dwarf galaxies orbit in a disc around the giant galaxy Andromeda, it looks like there must be something about how these galaxies formed or subsequently evolved that has led them to trace out this peculiar coherent structure,” said Professor Lewis. “Dwarf galaxies are the most numerous galaxy type in the universe, so understanding why and how they form this disc around the giant galaxy is expected to shed new light on the formation of galaxies of all masses.”
The researchers at the University of Bonn and the University of California at Irvine focused on a relationship called radial acceleration relation (RAR). In disk galaxies, stars move in circular orbits around the galactic center. The acceleration that forces them to change direction is caused by the attraction of matter in the galaxy. The RAR describes the relationship between this acceleration and the one caused by the visible matter only. It provides an insight into the structure of galaxies and their matter distribution
This difference makes the satellites a powerful probe for testing whether dark matter really exists. The Gaia spacecraft, which was launched by the European Space Agency (ESA) in 2013, could provide an answer. It was designed to study the stars in the Milky Way and its satellite galaxies in unprecedented detail and has collected a large amount of data.
However, it will probably take years to analyze the data. “Individual measurements are not enough to test the small differences we have found in our simulations,” explains doctoral student Enrico Garaldi. “But repeatedly examining the same stars improves the measurements every time. Sooner or later, it should be possible to determine whether the dwarf galaxies behave as in a universe with dark matter—or not.”
This question is one of the most pressing issues in cosmology today. The existence of dark matter was suggested more than 80 years ago by the Swiss astronomer Fritz Zwicky. He realized that galaxies move so fast within galaxy clusters that they should actually drift apart. He therefore postulated the presence of invisible matter which, due to its mass, exerts sufficient gravity to keep galaxies on their observed orbits. In the 1970s, his U.S. colleague Vera Rubin discovered a similar phenomenon in spiral galaxies like the Milky Way—they rotate so quickly that the centrifugal force should tear them apart if only visible matter was present.
Today, most physicists are convinced that dark matter makes up about 80 percent of the mass in the universe. Since it does not interact with light, it is invisible to telescopes. Yet, assuming its existence provides an excellent fit to a number of other observations—such as the distribution of background radiation, the afterglow of the Big Bang.
Dark matter also provides a good explanation for the arrangement and formation rate of galaxies in the universe. However, despite numerous experimental efforts, there is no direct proof that dark matter exists. This led astronomers to the hypothesis that the gravitational force itself might behave differently than previously thought.
According to the theory called modified Newtonian dynamics (MOND), the attraction between two masses obeys Newton’s laws only up to a certain point. At very small accelerations, such as those prevailing in galaxies, gravity becomes considerably stronger. Therefore, galaxies do not tear apart due to their rotational speed and the MOND theory can dispense with dark matter.
The Daily Galaxy via University of Bonn
Image credit top of page: With thanks to Astroplanets.com