The Milky Way would have a mass between four and five times less than that calculated until now, according to a study published on Wednesday, whose conclusions revolutionize knowledge about the characteristics of our galaxy.
This result is “the result of the ‘Gaia revolution’,” explains astronomer François Hammer, co-author of the study published in Astronomy and Astrophysics. Gaia, a satellite dedicated to mapping the Milky Way, delivered the positions and movements of 1.8 billion stars in its latest 2022 catalog.
This is a tiny fraction of the total that our spiral galaxy contains, a disk about 100,000 light years in diameter, formed by four large arms – one of which houses our solar system – that extend around an extraordinarily luminous center.
A “reevaluation of the mass”
The study of the Gaia catalog 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 the celestial bodies rotate around the center of the galaxy.
Observations of spiral galaxies had until now concluded that this curve was “flat”, meaning that once it reached a certain distance from the center, the rotation speed was constant.
But here, “it is the first time that we discover that beyond its disk, the curve descends,” explains François Hammer, “as if there were not much material” between 50 and 80,000 years from the galactic center.
With the consequence of a “reevaluation 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.
The international team’s study, carried out by astronomers from the Paris Observatory and the CNRS, has a second important consequence. This “calls into question the relationship between luminous matter and dark matter,” continues the astronomer.
“Not entirely well-founded” conclusions?
This hypothetical dark matter is also called dark matter because it is so far invisible and undetectable. It is assumed to provide the mass necessary for the cohesion of galaxies and represents 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 proportion significantly smaller, with only three times more dark matter than luminous matter.
Conclusions that the astronomer Françoise Combes, although a colleague of François Hammer at the Paris Observatory, considers “a little too daring” or even “perhaps not entirely well founded.”
In particular, because the study focuses on a small radius of the galaxy, while astronomers generally calculate the mass of the galaxy taking into account much greater 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 the same corresponding mass, points out Françoise Combes, a leading specialist in galaxy evolution. However, she welcomes “very precise work, which improves our knowledge of stars and their rotation”, about 80,000 light years from the galactic center.
A galaxy that evolves “silently and calmly”
François Hammer took into account the objections of Françoise Combes: he then clarified that “the mass (of the galaxy) calculated in the study is the mass by which we know that the stars of the disk are in equilibrium”, that is to say at a radius of 80,000 Light years. And he readily admits the “presence of external matter, and in particular hot gas,” capable of filling the Milky Way.
François Hammer’s team defends this work by defending the unique character of our galaxy. Unlike a large number of spiral galaxies, which experienced violent intergalactic collisions six billion years ago, the Milky Way has “evolved much more quietly and calmly for nine billion years,” according to François Hammer.
Another possibility to justify the difference between the Milky Way and other spiral galaxies: the method of observation. Which depends on the stars for the first and the gas clouds for the second.
Meanwhile, for Françoise Combes, the Milky Way “is nothing exceptional”, and as for dark matter, “it is like the others.”
Source: BFM TV
