According to a study published on Wednesday, the Milky Way would have a mass four to five times smaller than previously calculated, the conclusions of which upend knowledge about the properties of our galaxy.
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This result is the “fruit of the “Gaia revolution”,” explains astronomer François Hammer, co-author of the study published in Astronomy and Astrophysics, to AFP. Gaia, a satellite used to map the Milky Way, provided the positions and movements of 1.8 billion stars in its latest catalog in 2022.
Or a tiny fraction of what’s contained in our spiral galaxy, a disk about 100,000 light-years across that consists of four large arms – one of which hosts our solar system – that extend around an exceptionally luminous object.
Studying the Gaia catalog made it possible to calculate the Milky Way’s rotation curve with unprecedented precision, according to the study authors. The exercise consists of determining the speed at which celestial bodies rotate around the center of the galaxy.
Observations of spiral galaxies have so far come to the conclusion that this curve is “flat”, that is, from a certain distance from the center the rotation speed is constant.
But here “we discover for the first time that the curve slopes away beyond its disk,” 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 “reassessment of the mass of our Milky Way to values that are considered extremely low”, on the order of 200 billion solar masses, five times less than previously assumed.
“Daring conclusions”
The study by the international team, carried out by astronomers from the Paris Observatory and the CNRS, has a second important consequence. It “calls into question the relationship between luminous matter and dark matter,” the astronomer continues.
This hypothetical dark matter is also called dark matter because it is currently invisible and undetectable. It is said to provide the mass necessary to hold galaxies together and represents around six times the mass of the luminous matter from stars and gas clouds. For the Milky Way, the study’s calculations assume that this ratio is significantly lower and that there is 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, assessed to AFP as “a little too daring” or even “perhaps not entirely well-founded”.
Especially because the study focuses on a smaller radius of the galaxy, while astronomers generally calculate the mass of the galaxy taking much larger distances into account.
But in addition to gases, globular clusters, dwarf galaxies or even the Magellanic Cloud, “up to these distances we have a lot of dark matter” and just as much corresponding mass, notes Françoise Combes, a leading specialist in the evolution of galaxies.
However, she welcomes “very precise work that improves our knowledge of the stars and their rotation” up to about 80,000 light-years from the galactic center.
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 calmly and serenely over the last nine billion years,” according to Hammer.
Another way to explain the difference between the Milky Way and other spiral galaxies: the observation method. Which in the first case is based on the stars and in the second on the gas clouds.
Meanwhile, for Françoise Combes, the Milky Way is “not at all exceptional,” and as far as dark matter is concerned, “it is like the others.”