A study published on Wednesday reported that the mass of the Milky Way Galaxy is four to five times less than previously thought, conclusions that upend the data that was known until now about the galaxy that includes planet Earth.
This result is “the fruit of the Gaia revolution,” as astronomer François Hammer, co-author of the study published by the journal “Astronomy and Astrophysics,” explained to AFP. Gaia, the satellite dedicated to mapping the Milky Way Galaxy, revealed the positions and movements of 1.8 billion stars, in its latest data in 2022.
Size of the Milky Way Galaxy
This represents a small fraction of the total contents of our spiral galaxy, which is a disk about 100,000 light-years in diameter, consisting of four large arms, one of which includes our solar system, all extending around a very luminous center.
Studying Gaia data made it possible to calculate the Milky Way’s rotation curve with unprecedented accuracy, according to the study’s authors. The task is to determine the speed at which celestial bodies revolve around the center of the galaxy.
Observations of spiral galaxies had previously concluded that this curve was “flat,” meaning that once a certain distance from the center was reached, the speed of rotation was constant.
But “this is the first time that we discover that the curve descends outside its disk,” François Hammer said, “as if there is not a lot of matter” at a distance of between 50 and 80 thousand years from the center of the galaxy.
As a result, the mass of our Milky Way Galaxy was recalibrated to values considered very low, about 200 billion times the mass of the Sun, five times less than previous estimates.
“Bold conclusions”
In a related context, the study conducted by the international team and led by astronomers from the Paris Observatory and the National Center for Scientific Research in France has a second major result, as it calls into question “the relationship between luminous matter and dark matter,” according to the astronomer.
This hypothetical dark matter is also called dark matter because it has remained invisible and undetectable until now. It is supposed to provide the mass necessary for the cohesion of galaxies, and represents about 6 times the mass of luminous matter, consisting of stars and gas clouds.
As for the Milky Way Galaxy, the study calculates that this percentage is much lower, with there being only three times more dark matter than bright matter.
But astronomer Françoise Combes, a colleague of François Hammer at the Paris Observatory, considered, via Agence France-Presse, that these conclusions are “a bit bold,” or even “perhaps not well-founded.”
This is notably because the study focuses on low galaxy radii, while astronomers generally calculate galaxy mass taking into account much larger distances.
However, in addition to gas, globular star clusters, dwarf galaxies or even the Magellanic Cloud, “we have a lot of dark matter up to these distances,” with a similar mass, notes Françoise Combe, a senior specialist in galaxy evolution.
But Coombe welcomes “very 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.
