Image: The rotation speed of the Milky Way is represented as a function of the distance from the galactic center. White dots and error bars represent measurements obtained from the Gaia DR3 catalog. The blue curve represents the best fit including ordinary matter and dark matter. The end of the curve in orange shows the Keplerian decay, which begins beyond the optical disk of our Galaxy indicating the absence of matter.
© Jiao, Hammer et al. / Observatoire de Paris – PSL / CNRS / ESA / Gaia / ESO / S. Brunier
To measure the masses of galaxies we use a unit which corresponds to the mass of our Sun (M☉). Galaxies weigh between a few million solar masses and a few trillion solar masses.
Until now, the Milky Way was estimated to weigh about 1012 M☉. Part of this mass, made up of ordinary matter (stars, gas and dust), is estimated at approximately 0.6 x 1011 M☉. The rest of the Milky Way's mass, at least 6 times greater, is made up of dark matter, a hypothetical matter that is both invisible and sensitive to gravitation.
In the 1970s, Vera Rubin (1928-2016) showed that the rotation speed of stars on the outskirts of spiral galaxies remained constant, which did not correspond to the predictions of Newton's laws. The rotation curves should have followed a so-called “Keplerian” decay.
The direct consequence of this discovery was to propose the existence of dark matter, tirelessly sought after since.
According to a study published on September 27, 2023 in the European scientific journal “Astronomy and Astrophysics”, the mass of the Milky Way would be four to five times lower than the latest estimates.
The exact mass obtained from data in the Gaia DR3 catalog would be 2.06 x 1011 solar masses compared to 1012 previously. This study was carried out by astronomers from the Paris Observatory and the CNRS (France), Leroy, N., Martin, N. F., Wegg, C., et al. (2023).
The 2023 study takes into account the movements of 1.8 billion stars provided by the Gaia satellite with unprecedented precision. The rotation curve of the Milky Way clearly shows, with a probability of 99.7%, a Keplerian decay and not a constant rotation speed as previously.
Now many questions arise in cosmology!
Why do other large spiral galaxies not exhibit a rotation curve with a Keplerian decay?
Why would our galaxy be special?
Scientists have put forward two reasons:
- The Milky Way is a spiral galaxy that has experienced few disturbances linked to collisions between galaxies. The last one occurred about 9 billion years ago, when the dwarf galaxy SagDEG was absorbed by the Milky Way. The average for spiral galaxies is 6 billion years.
The Milky Way is located in the Local Group, a group of galaxies that includes about 50 galaxies. The Local Group is a relatively quiet region of the Universe, and collisions between galaxies are relatively rare there.
- The rotation curve of a galaxy is a graphical representation of the rotation speed of stars or gas as a function of their distance from the center of the galaxy. There is a big methodological difference between the new rotation curve of our Galaxy obtained from the movement of the stars (data delivered by the Gaia satellite), and the measurements made from neutral gas (hydrogen and helium) for the other galaxies. .
In conclusion, it is necessary to re-evaluate the rotation curves of large spiral galaxies and the quantities of ordinary matter compared to dark matter.
Why are Galaxies, Unlike Stars, So Close to Each Other? 
Galaxies
of the Local Group 
The hidden galaxy, one of Euclid's first images 
The Virgo Cluster spans approximately three Full Moons 
Merging galaxies and black holes 
Mirages created by gravitational lenses 
Mystery of the Big Bang, the problem of the horizon 
Cartwheel Galaxy Cosmic Event 
The first second of our history 
A small galaxy tears apart the large NGC 6745 
The mystery of gamma bursts 
The Cigar Explosion 
Shockwaves
Gould's belt, a stellar fireworks display 
Recombination
in cosmology 
Journey to the center of our galaxy 
Lyman-alpha
bubbles 
Andromeda in the ultraviolet 
The most beautiful galaxy clusters 
Tinkerbell merger of three galaxies 
A gigantic black hole 
Enigma
of coplanar galaxies 
The cluster of galaxies Coma in its soup 
The cannonball, proof of dark matter 
El Gordo galaxy cluster 
Einstein
ring and cross 
How to measure distances in the Universe? 
The Hubble sequence and types of galaxies 
The spiral shape of the galactic arms 
Even more stars, the Cigar galaxy 
The
Universe of X-rays 
The most beautiful galaxies 
The oldest galaxies in the universe 
Quasars
the nuclei of galaxies 
Sagittarius A black hole at the center of our Galaxy 
The MOND theory and its contradiction 
The first image of a black hole 
Central
area of the Milky Way 
Laniakea, our supercluster of galaxies