Astronomers using NASA and ESA (European Space Agency) telescope data have released a new map of all the skies of the most distant region of our galaxy.
Known as the galactic halo, this area lies outside the swirling spiral arms that make up the recognizable central disk of the Milky Way and is sparsely populated by stars. Although the halo may seem mostly empty, it is also predicted to contain a massive reservoir of dark matter, a mysterious and invisible substance thought to make up the bulk of all mass in space.
The data for the new map comes from ESA’s Gaia mission and NASA’s Infrared Survey Explorer of a wide-range near-Earth object, or NEOWISE, which operated from 2009 to 2013 under the name 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 larger than two dwarf galaxies orbiting the Milky Way – sailed through the Milky Way’s galactic halo like a ship through water, creating gravity waking up in the stars behind it. The LMC is located about 160,000 light-years from Earth and is less than a quarter of the mass of the Milky Way.
Although the inner parts of the halo are mapped with a high level of precision, this is the first map to give a similar picture of the outer regions of the halo, where the vigil is located – about 200,000 light-years to 325,000 light-years from the galactic center. Previous studies have suggested the existence of a vigil, but a map of the sky confirms its presence and offers a detailed view of its shape, size, and location.
A simulation of the dark matter surrounding the Milky Way galaxy (the small ring in the center) and the Large Magellanic Cloud (LMC) reveals two high-density areas: less than two light blue areas is a sign created by the LMC moving through this area. The larger one corresponds to the excess of stars in the northern hemisphere of the Milky Way. Credit: NASA / JPL-Caltech / NSF / R. Boli / N. Garavito-Camargo and G. Besla
This halo disorder also gives astronomers the opportunity to study something they cannot 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 thought to create a scaffold on which galaxies are built, so that without it, galaxies would disintegrate as they rotate. Dark matter is estimated to be five times more common in space than all matter that emits and / or communicates with light, from stars across planets to gas clouds.
Although there are several theories about the nature of dark matter, they all suggest that it should be present in the halo of the Milky Way. If this is the case, while the LMC would be sailing through this region, it should leave a trail in dark matter as well. Observation on a new stellar map is thought to be the outline of this passage of dark matter; the stars are like leaves on the surface of this invisible ocean, whose position changes with 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 gravity of dark matter pulls on the LMC and slows it down. This will cause the dwarf galaxy’s orbit to shrink and shrink, until the galaxy finally collides with the Milky Way in about 2 billion years. These types of mergers can be a key driver of the growth of massive galaxies across the universe. In fact, astronomers think the Milky Way merged with another small galaxy about 10 billion years ago.
“This robbery of the energy of a smaller galaxy is not only the reason why the LMC merges with the Milky Way, but also why all galaxy mergers happen,” said Rohan Naidu, a doctoral student in astronomy at Harvard University and co-author of the new paper. “Waking up on our map is really good confirmation that our basic picture is how galaxies merge in place!”
A rare opportunity
The authors also believe that the new map – along with additional data and theoretical analyzes – can provide a test for various theories about the nature of dark matter, such as whether it consists of particles, such as regular matter, and what the properties of those particles are.
“You can imagine that the vigil behind the boat will be different if the ship is sailing through water or through 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 awakening are determined by which theory of dark matter we apply.”
Conroy led a team that mapped the positions of over 1,300 stars in the halo. The challenge arose in trying to measure the exact distance from Earth to much of these stars: It is often impossible to decipher whether a star is faint and near or bright and distant. The team used data from ESA’s Gaia mission, which provides the location of many stars in the sky but cannot measure the distance to stars in the outer regions of the Milky Way.
After identifying the stars that were most likely in the halo (because they were obviously not inside our galaxy or LMC), the team searched for stars belonging to the class of giant stars with a specific light “signature” that NEOWISE can detect. Knowing the basic properties of the selected stars allowed the team to determine the distance from Earth and create a new map. It plots a region that begins about 200,000 light-years from the center of the Milky Way, or roughly where the LMC awakening is predicted to begin, and extends about 125,000 light-years beyond that.
Conroy and his colleagues were inspired to hunt for the LMC after learning about a team of astrophysicists from the University of Arizona at Tucson who predict computer models of what dark matter in the galactic halo should look like. The two groups worked together on a new study.
One Arizona team model, included in the new study, predicted the general structure and specific location of stellar traces discovered on the new map. Once the data confirmed that the model was accurate, the team could confirm what other investigations suggested: that the LMC was probably in its first orbit around the Milky Way. If a smaller galaxy had already made more orbits, the shape and location of the vigil would have been significantly different from what was observed. Astronomers think that the LMC formed in the same environment as the Milky Way and another nearby galaxy, M31, and that it is nearing the end of a long first orbit around our galaxy (about 13 billion years ago). Its next trajectory will be much shorter due to its interaction with the Milky Way.
“Confirmation of our theoretical prediction by observational data tells us that our understanding of the interaction between these two galaxies, including dark matter, is on track,” said Nicolás Garavito-Camargo, a doctoral student in astronomy at the University of Arizona, who led the work on the model used. .
The new map also gives astronomers a rare opportunity to test the properties of dark matter (imaginary water or honey) in our own galaxy. In the new study, Garavito-Camargo and colleagues used a popular theory of dark matter called cold dark matter, which corresponds relatively well to the observed map of stars. Now a team from the University of Arizona is conducting simulations that use different theories of dark matter to determine which one best matches the vigilance observed in the stars.
“It’s really a special set of circumstances that came together to create this scenario that allows us to test our theories of dark matter,” said Gurtina Besla, co-author of the study and an associate professor at the University of Arizona. “But we can only accomplish that test by combining this new map and the dark matter simulations we made.”
Launched in 2009, the WISE spacecraft went into hibernation in 2011 after completing its primary mission. In September 2013, NASA reactivated the spacecraft with the primary goal of scanning objects near Earth or NEO, and the mission and spacecraft were renamed NEOWISE. NASA’s Jet Propulsion Laboratory in Southern California managed and administered WISE for NASA’s Directorate of Science Mission. The mission was selected competitively under NASA’s Researchers program run by the 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 Office of Planetary Defense Coordination.
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