Astronomers using data from NASA and ESA (European Space Agency) telescopes have released a new all-sky map of the outermost region of our galaxy. [Editor’s note: See Related Multimedia link below.] Known as the galactic halo, this area lies outside the swirling spiral arms that form the recognizable central disk of the Milky Way and is sparsely populated with stars. While the halo may appear nearly empty, it is also predicted to contain a massive reservoir of dark matter, a mysterious and invisible substance believed to make up the bulk of all mass in the universe.
The data for the new map comes from ESA’s Gaia mission and NASA’s Near Earth Object Wide Field Infrared Survey Explorer, or NEOWISE, which operated from 2009 to 2013 as WISE. The study uses data collected by the spacecraft between 2009 and 2018.
The new map reveals how a small galaxy called the Large Magellanic Cloud (LMC) – so named because it is the larger of the two dwarf galaxies orbiting the Milky Way – crossed the galactic halo of the Milky Way as a ship in the water, its gravity creating a wake in the stars behind it. The LMC is located about 160,000 light years from Earth and represents less than a quarter of the Milky Way’s mass.
Although the inner parts of the halo have been mapped to a high level of accuracy, it is the first map to provide a similar picture of the outer regions of the halo, where the wake is located – approximately 200,000 light years to 325,000 light years. from the galactic center. Previous studies have hinted at the existence of the wake, but the entire sky map confirms its presence and offers a detailed view of its shape, size, and location.
This halo disruption also allows astronomers to study something they can’t directly observe: dark matter. Although it does not emit, reflect or absorb light, the gravitational influence of dark matter has been observed throughout the universe. It is believed to create a scaffolding that galaxies are built on, so that without it the galaxies would spin apart. Dark matter is estimated to be five times more common in the universe than all matter that emits and / or interacts with light, from stars to planets to gas clouds.
While there are several theories about the nature of dark matter, all of them indicate that it should be present in the Milky Way halo. If so, as the LMC navigates this region, it should also leave a wake in dark matter. The wake observed in the new star map is believed to be the outline of this dark matter wake; stars are like leaves on the surface of this invisible ocean, their position moving with dark matter.
The interaction between dark matter and the Large Magellanic Cloud has big implications for our galaxy. As the LMC revolves around the Milky Way, the gravity of dark matter drags on the LMC and slows it down. This will shrink the orbit of the dwarf galaxy further and further, until the galaxy finally collides with the Milky Way in about 2 billion years. These types of mergers could be a key driver of the growth of massive galaxies across the universe. In fact, astronomers believe the Milky Way merged with another small galaxy around 10 billion years ago.
“This flight of energy from a small galaxy is not only why the LMC merges with the Milky Way, but also why all galaxy mergers are happening, “said Rohan Naidu, a doctoral student in astronomy at Harvard University and co-author of the new article.” The wake on our map is a very nice confirmation that our base galaxy fusion image is on point! “
A rare opportunity
The authors of the article also believe that the new map – with additional data and theoretical analysis – may provide a test for different theories about the nature of dark matter, for example whether it is made up of particles, like regular matter. , and what the properties of these particles are.
“You can imagine the wake behind a boat will be different whether the boat is sailing in water or in honey,” said Charlie Conroy, professor at Harvard University and 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 over 1,300 stars in the halo. The challenge has come in trying to measure the exact distance from Earth to a large part of these stars: it is often impossible to determine whether a star is faint and near or bright and far. The team used data from ESA’s Gaia mission, which provides the locations of many stars in the sky, but cannot measure distances to stars in outer regions of the Milky Way.
After identifying stars that were most likely located in the halo (as they were not clearly inside our galaxy or the LMC), the team looked for stars belonging to a class of giant stars with a “signature “specific light 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 traces an area starting about 200,000 light years from the center of the Milky Way, or roughly where the wake of the LMC was to begin, and extending about 125,000 light years beyond.
Conroy and his colleagues were inspired by CML’s wake research after discovering a team of astrophysicists at the University of Arizona in Tucson who are making computer models predicting what dark matter should look like in the galactic halo. The two groups worked together on the new study.
A model from the Arizona team, included in the new study, predicted the general structure and specific location of the star wake revealed in the new map. Once the data confirmed the model was correct, the team was able to confirm what other surveys have also suggested: that the LMC is likely in its first orbit around the Milky Way. If the smaller galaxy had already made several orbits, the shape and location of the wake would be significantly different from what has been observed. Astronomers believe that the LMC formed in the same environment as the Milky Way and another neighboring galaxy, M31, and that it is about to complete a long first orbit around our galaxy (about 13 billion d ‘years). Its next orbit will be much shorter due to its interaction with the Milky Way.
“Confirming our theoretical prediction with observational data tells us that our understanding of the interaction between these two galaxies, including dark matter, is on the right track,” said Nicolás Garavito-Camargo, doctoral student in astronomy at the University of Arizona, who led work on the model used in the article.
The new map also offers astronomers a rare opportunity to test the properties of dark matter (fictitious water or honey) in our own galaxy. In the new study, Garavito-Camargo and his colleagues used a popular dark matter theory called cold dark matter that matches the observed star map relatively well. Now, the University of Arizona team is running simulations that use different dark matter theories to see which best matches the observed wake in stars.
“It’s a really special set of circumstances that came together to create this scenario that allows us to test our dark matter theories,” said Gurtina Besla, study co-author and associate professor at the University. from Arizona. “But we can only do this test with the combination of this new map and the dark matter simulations we’ve built.”
Launched in 2009, the WISE spacecraft was put into hibernation in 2011 after completing its main mission. In September 2013, NASA reactivated the spacecraft with the primary purpose of searching for NEOs, or NEOs, and the mission and spacecraft were renamed NEOWISE. NASA’s Jet Propulsion Laboratory in Southern California managed and operated WISE for the NASA Science Mission Directorate. The mission was selected as part of NASA’s Explorers program run 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 the NASA Planetary Defense Coordination Office.