The Milky Way would have a mass four to five times lower than that calculated so far, according to a study published Wednesday, the conclusions of which shake up knowledge of the characteristics of our galaxy.
This result is the “fruit of the” revolution Gaia”, explains to AFP astronomer François Hammer, co-author of the study published in Astronomy and Astrophysics. Gaiaa satellite dedicated to mapping the Milky Way, delivered the positions and movements of 1.8 billion stars, in its latest catalog in 2022.
That’s a tiny fraction of the total contained in our spiral galaxy, a disk some 100,000 light years in diameter, made up of four large arms – one of which houses our solar system – which stretch around an extraordinarily luminous center .
Study of the catalog of Gaia made it possible to calculate the rotation curve of the Milky Way with unprecedented precision, according to the authors of the study. The exercise consists of establishing the speed at which celestial bodies rotate around the center of the galaxy.
Observations of spiral galaxies had until now concluded that this curve was “flat”, that is to say that once it reached a certain distance from the center, the rotation speed was constant.
But here, “this is the first time that we discover that beyond its disk, the curve falls”, explains François Hammer, “as if there was not much material” between 50 and 80,000 years from the galactic center.
With the consequence of a “reassessment of the mass of our Milky Way to values considered extremely low”, of the order of 200 billion times the mass of the Sun, five times less than previously estimated.
“Daring conclusions”
The study by the international team, carried out by astronomers from the Paris Observatory and the CNRS, has a second major consequence. It “calls into question the relationship between luminous matter and dark matter,” continues the astronomer.
This hypothetical dark matter is also called dark matter because it is so far invisible and undetectable. It is supposed to provide the mass necessary for the cohesion of galaxies, and represent approximately six times the mass of luminous matter, made up of stars and gas clouds. For the Milky Way, the study’s calculations see this ratio significantly lower, with only three times more dark matter than luminous matter.
Conclusions that astronomer Françoise Combes, although a colleague of François Hammer at the Paris Observatory, judges to AFP to be “a little too daring”, or even “perhaps not entirely well-founded”.
Notably because the study focuses on a reduced radius of the galaxy, while astronomers, in general, calculate the mass of the galaxy 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 as much corresponding mass, notes Françoise Combes, a leading specialist in evolution. galaxies.
However, she welcomes “very precise work, which improves our knowledge of the stars and their rotation”, up to approximately 80,000 light years from the galactic center.
Mr. Hammer took into account the objections of Françoise Combes: he later clarified to AFP that “the mass (of the galaxy) calculated in the study is the mass for which we know that the stars of the disk are at the ‘equilibrium’, that is to say within a radius of 80,000 light years. And he readily admits the “presence of matter outside, and in particular hot gas”, capable of filling the Milky Way.
François Hammer’s team defends this work by arguing for the unique character of our galaxy. Unlike many spiral galaxies, which experienced violent collisions between galaxies six billion years ago, the Milky Way has “evolved much more quietly and calmly over the past nine billion years”, according to Mr. Hammer.
Another possibility to justify the difference between the Milky Way and other spiral galaxies: the method of observation. Which relies on the stars for the first, and on the gas clouds for the second.
In the meantime, for Françoise Combes, the Milky Way “is not exceptional at all”, and as for dark matter, “it is like the others”.