The Milky Way’s mass is expected to be four to five times less than that calculated so far, according to a study published on Wednesday, whose conclusions shake up knowledge about the properties of our galaxy.
This result is “the fruit of the Gaia revolution,” as astronomer François Hammer, co-author of the study published in the journal Astronomy and Astrophysics, explains to AFP. Gaia, the satellite dedicated to mapping the Milky Way, revealed the positions and movements of 1.8 billion stars, in its latest catalog in 2022.
Or a small fraction of the total contained in our spiral galaxy, a disk about 100,000 light-years in diameter, made up of four large arms – one of which includes our solar system – that extend around an extremely luminous center.
Studying the Gaia catalog allowed the Milky Way’s rotation curve to be calculated with unprecedented accuracy, according to the study’s authors. The exercise consists of determining the speed at which celestial bodies revolve around the center of the galaxy.
Observations of spiral galaxies so far have concluded that this curve was “flat,” meaning that once it reached a certain distance from the center, the speed of rotation was constant.
But here “this is the first time we discover that outside its disk, the curve falls off,” explains François Hammer, “as if there was not much matter” between 50 and 80 thousand years from the center of the galaxy.
As a result, “the mass of our Milky Way Galaxy has been re-evaluated to values considered very low,” at about 200 billion times the mass of the Sun, five times less than previous estimates.
The study by the international team, conducted by astronomers from the Paris Observatory and the Center National de la Recherche Scientifique, has a second key finding. The astronomer continues: “This calls into question the relationship between luminous matter and dark matter.”
This hypothetical dark matter is also called dark matter because it is invisible and undetectable so far. It is supposed to provide the mass necessary to hold galaxies together, and represents about six times the mass of luminous matter, consisting of stars and gas clouds. For the Milky Way, the study calculates that this ratio is much lower, with only three times more dark matter than bright matter.
The conclusions that astronomer Françoise Coombs, although a colleague of François Hamer at the Paris Observatory, told AFP “a bit bold”, or even “perhaps not well-founded”.
This is notably because the study focuses on a low galaxy radius, while astronomers generally calculate galaxy mass taking into account much larger distances.
However, in addition to gases, globular clusters, dwarf galaxies or even the Magellanic Cloud, “we have a lot of dark matter up to these distances,” and a similar mass, notes Françoise Coombes, a leading specialist in galaxy evolution.
But she welcomes “extremely precise work that improves our knowledge of stars and their rotation,” up to a distance of about 80,000 light-years from the galactic center.
François Hammer’s team defends this work by defending the uniqueness of our galaxy. Unlike many spiral galaxies, which experienced violent collisions between galaxies six billion years ago, the Milky Way “evolved more calmly and quietly for nine billion years,” according to Mr. Hammer.
Another possibility to explain the difference between the Milky Way and other spiral galaxies: the method of observation. Which depends on the stars in the first, and on gas clouds in the second.
Meanwhile, Françoise Coombes believes that the Milky Way is “not exceptional at all,” and as for dark matter, “it is like the others.”
