While there are multiple theories about the nature of dark matter, they all indicate that it should be present in the halo of the Milky Way. If so, the LMC must also leave a wake in the dark matter as it travels through this region. The wake seen in the new star map is believed to be the outline of this dark matter wake; the stars are like leaves on the surface of this invisible ocean, shifting their position with the dark matter.
The interaction between dark matter and the Large Magellanic Cloud has major implications for our galaxy. As the LMC orbits the Milky Way, the dark matter’s gravity drags and slows the LMC. As a result, the orbit of the dwarf galaxies will become smaller and smaller, until the Milky Way will eventually collide with the Milky Way in about 2 billion years. These kinds of mergers can be a major driver for the growth of huge galaxies in the universe. In fact, astronomers think that the Milky Way merged with another small galaxy about 10 billion years ago.
“This depriving the energy of a smaller galaxy is not only why the LMC is merging with the Milky Way, but also why all galaxy mergers are happening, ”said Rohan Naidu, a graduate student of astronomy at Harvard University and co-author of the new paper. “The wake on our map is really nice confirmation that our basic picture of how galaxies merge is clear!”
A rare opportunity
The authors of the paper also think that the new map – along with additional data and theoretical analyzes – could test different theories about the nature of dark matter, such as whether it is made up of particles, such as ordinary matter, and what its properties are. those are particles.
“You can imagine that the wake behind a boat will be different if the boat travels through water or honey,” said Charlie Conroy, a professor at Harvard University and an astronomer at the Center for Astrophysics | Harvard & Smithsonian, who co-authored the study. “In this case, the properties of the wake are determined by the dark matter theory we apply.”
Conroy led the team that mapped the positions of more than 1,300 stars in the halo. The challenge arose by trying to measure the exact distance from Earth to many of those stars: it is often impossible to tell whether a star is faint and close or bright and far away. The team used data from ESA’s Gaia mission, which provides the location of many stars in the sky, but cannot measure distances to the stars in the outer regions of the Milky Way.
After identifying stars that were likely in the halo (because they weren’t clearly located in our galaxy or the LMC), the team looked for stars that belonged to a class of giant stars with a specific light signature detectable by NEOWISE. Knowing the basic properties of the selected stars allowed the team to determine their distance from Earth and create the new map. It maps a region that begins about 200,000 light-years from the center of the Milky Way, or roughly where the LMC’s wake would begin, and extends beyond it about 125,000 light-years.
Conroy and his colleagues were inspired to look for the wake of LMC after hearing about a team of astrophysicists at the University of Arizona in Tucson creating computer models that predict what dark matter in the galactic halo should look like. The two groups collaborated on the new study.
A model from the Arizona team, included in the new study, predicted the overall structure and specific location of the star’s wake revealed in the new map. Once the data confirmed the model was correct, the team was able to confirm what other studies pointed out as well: that the LMC is likely in its first orbit around the Milky Way. If the smaller galaxy had already made multiple orbits, the shape and location of the wake would differ significantly from what has been observed. Astronomers think the LMC formed in the same environment as the Milky Way and another nearby galaxy, M31, and nearly completed a long first orbit around our galaxy (about 13 billion years). His next job will be much shorter because of his interaction with the Milky Way.
“If we confirm our theoretical prediction with observational data, we know that our understanding of the interaction between these two galaxies, including dark matter, is on the right track,” said Nicolás Garavito-Camargo, a PhD student in astronomy at the University of Arizona. who led the work on the model used in the newspaper.
The new map also offers astronomers a rare opportunity to test the properties of the dark matter (the imaginary water or honey) in our own galaxy. In the new study, Garavito-Camargo and colleagues used a popular dark matter theory called cold dark matter that fits relatively well with the observed star map. Now, the University of Arizona team is running simulations that use different dark matter theories to see which one best matches the wake observed in the stars.
“It’s a very special set of conditions that came together to create this scenario that will allow us to test our dark matter theories,” said Gurtina Besla, a co-author of the study and an associate professor at the University of Arizona. “But we can only realize that test with the combination of this new map and the dark matter simulations we have built.”
Launched in 2009, the WISE spacecraft was hibernated in 2011 after completing its primary mission. In September 2013, NASA reactivated the spacecraft with the primary purpose of scanning for near-Earth objects, or NEOs, and the mission and spacecraft were renamed NEOWISE. NASA’s Jet Propulsion Laboratory in Southern California managed and operated WISE for NASA’s Science Mission Directorate. The mission was competitively selected under NASA’s Explorers Program, administered by the agency’s Goddard Space Flight Center in Greenbelt, Maryland. NEOWISE is a project of JPL, a division of Caltech, and the University of Arizona, supported by NASA’s Planetary Defense Coordination Office.