The revisited mass of the Milky Way is much smaller than expectations from cosmology

An international team lead by astronomers from the Paris Observatory–PSL and the CNRS has determined the mass of the Milky Way with the highest degree of accuracy possible according to the most recent Gaia satellite catalog from the European Space Agency (ESA). This work raises significant cosmological problems, especially about the mass of dark matter in our galaxy.

The estimated total mass of the Milky Way is just 2.06 x 1011 solar masses, or 200 billion times smaller than the sun. This is a dramatic downward revision, with the estimate being four to five times lower than the prior estimations.

The source of this new value came from the third data release of the Gaia catalog, which was released in 2022. This catalog contains detailed information on 1.8 billion stars, including all three spatial components and all three velocity components in a six-dimensional Milky Way region.

The Milky Way's tolerably thin layer

Scientists were able to determine the mass of our galaxy and create the most exact rotation curve yet seen for a spiral galaxy—in this case, our own galaxy—using the Gaia data. Until Gaia, it was difficult to acquire a stable rotation curve for our galaxy, in contrast to exterior spiral galaxies. This problem resulted from our location inside the Milky Way, which obscured our ability to determine the exact positions and velocities of stars inside the galactic disk.

Scientists discovered that our galaxy's rotation curve is unusual in that it is not flat, in contrast to those identified for other massive spiral galaxies, in their study, which was published on September 27, 2023, in the journal Astronomy and Astrophysics.

On the other hand, this curve starts to quickly diminish near the galaxy's periphery, in accordance with the Keplerian decline forecast.

Casting doubt on cosmology

In order to derive a rotation curve for the Milky Way that shows a Keplerian decrease, we must place our galaxy in a cosmic context.

Indeed, the discovery that the rotating velocities of the massive spiral galaxy disks were far faster than those predicted by a Keplerian decline was one of the most significant discoveries in contemporary astronomy. Astronomers Vera Rubin and Albert Bosma showed in the 1970s that spiral galaxies rotate at a constant speed long beyond their optical disks. Rubin employed measurements of ionized gas, while Bosma analyzed neutral gas.

The finding immediately led to the theory that spiral galaxy disks contain dark matter, which is mass that is not visible to the naked eye and is instead spread in a halo around them. The rotation curves would have followed a "Keplerian" fall in the absence of this dark matter. The latter suggests that there isn't much matter outside of the optical disk.

Provided by Paris Observatory