New Study Reveals Surprising Mass of the Milky Way Galaxy

A study published today (Wednesday) reported that the mass of the Milky Way Galaxy is four to five times less than previously thought, which are conclusions that turn upside down the data that was known until today about the galaxy that includes the 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 Agence France-Presse. 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.

This constitutes a small portion of the total contents of our spiral galaxy, which is a disk with a diameter of about 100,000 light-years, and consists 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,” according to François Hammer, “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 very low values,” about 200 billion times the mass of the Sun, five times less than previous estimates.

The Milky Way Galaxy appears in the sky of Gozo Island in Malta (Reuters)

“Bold conclusions”

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 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.

But astronomer Françoise Combes, a colleague of François Hammer at the Paris Observatory, said 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 Combe 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 center of the galaxy.

François Hammer’s team defends this work by talking up the uniqueness of our galaxy. Unlike a large number of spiral galaxies, which witnessed violent collisions between galaxies six billion years ago, the Milky Way “evolved more quietly for nine billion years,” according to Hammer.

Also, the difference between the Milky Way and other spiral galaxies can be justified by the method of observation, which relies on stars in the former, and on gas clouds in the latter.

Meanwhile, Françoise Combe believes that the Milky Way Galaxy “is not exceptional at all,” but in terms of dark matter, “it is like the others.”