Thanks to the latest catalog from ESA’s Gaia satellite, an international team led by astronomers from the Paris Observatory (PSL) and the CNRS obtains the most accurate measurement of the Milky Way’s mass. This study is the subject of a paper published September 27, 2023 in the journal Astronomy & Astrophysics and opens the door to important questions in cosmology, particularly the relative amount of dark matter contained in our galaxy.
The total mass of the Milky Way is estimated to be only 2.06 x 1011 solar masses. It is therefore reassessed downward by a factor four to five times smaller than previous estimates, which put it at 1012 solar masses.
Illustration image Pixabay
This new value was determined using data from the Gaia satellite’s third catalog released in 2022, which includes all three spatial components and the three velocity components for 1.8 billion stars within the Milky Way.
The lasting lightness of the Milky Way
Using this data, researchers were able to construct the most precise rotation curve (1) ever observed for a spiral galaxy, in this case ours, and deduce its mass (2).
Before Gaia, a robust rotation curve could not be determined for our galaxy, unlike external spiral galaxies. This is explained by our position within the Milky Way, which prevents us from accurately distinguishing the movements and distances of the stars that make up its disk.
In the study, which appeared on September 27, 2023 in the journal Astronomy & Astrophysics, the rotation curve of our galaxy turns out to be atypical: It is not flat, unlike all other large spiral galaxies.
Rotation curve of the Milky Way, depicting the circular rotation speed as a function of distance from the center. White dots and error bars represent measurements obtained from the Gaia DR3 catalog. The blue curve represents the best fit of the rotation curve by a model that includes ordinary matter and dark matter. The orange part of the curve shows the Kepler decay that begins beyond the optical disk of our galaxy. A constant speed is rejected with a probability of 99.7% (3σ).
© Jiao, Hammer et al. / Paris Observatory – PSL / CNRS / ESA / Gaia / ESO / S. Brunier
On the contrary, beyond the outer disk of the galaxy this curve begins to decline rapidly. Furthermore, this drop in velocity follows the so-called “Keplerian” prediction (3).
A whirlwind in cosmology
To obtain a Kepler decay rotation curve for the Milky Way, the object must be placed in a cosmological context.
In fact, one of the great discoveries of modern astronomy was to find that the motions around the large disks of spiral galaxies were much faster than would be expected from a Kepler decay. In the 1970s, astronomers Vera Rubin, using observations of ionized gas, and Albert Bosma, using neutral gas, showed that the rotation speed of spiral galaxies remained constant well beyond their optical disk.
The direct consequence of this discovery was to propose the existence of dark matter in addition to the observable matter distributed in a halo that surrounds the disks of spiral galaxies and accounts for the majority of the galactic mass. Without this dark matter, the rotation curves would have to follow a so-called “Keplerian” decay, indicating the absence of matter outside the optical disk.
So this is actually the case with the Milky Way. Since ordinary matter (stars and cold gas) is estimated to be just over 0.6 larger than ordinary matter.
This result therefore represents a revolution in cosmology, because until now it was agreed that dark matter must be at least six times more common than ordinary matter.
Two attempts at explanation
If almost all other large spiral galaxies do not have a rotation curve with a Kepler decay, why should ours be any different?
The first possibility could arise from the fact that the Milky Way is a galaxy that has experienced few disturbances related to violent collisions between galaxies, with the last one occurring about 9 billion years ago, compared to an average of 6 billion years for the spiral galaxies. In any case, this suggests that the rotation curve obtained for the Milky Way is particularly precise and is not influenced by the remnants of such an ancient collision.
The second possibility arises from the methodological difference between the rotation curve obtained from the data provided by the Gaia satellite in six dimensions and the measurements made for most other galaxies in neutral gas.
This work paves the way for a reassessment of the rotation curves of large spiral galaxies and their ordinary and dark matter content.
Remarks:
(1) A rotation curve gives the circular speed as a function of the radius (see figure above).
(2) The more massive a body is, the higher the speed of the objects that revolve around it in order to avoid falling.
(3) Satellites in orbit have speeds that follow the laws of universal attraction known as “Kepler’s Laws.” The further a satellite is from its main body, the slower its rotation speed is, as its distance entails less gravitational pull. This decrease in speed is called “Keplerian” and is observed, for example, in the planets of our solar system.
reference
The article “Detection of the Kepler Decline in the Milky Way Rotation Curve” by Yongjun Jiao et al. (2023) appears in the journal Astronomy & Astrophysics, September 27, 2023.
DOI: 10.1051/0004-6361/202347513
Article also available at: https://arxiv.org/abs/2309.00048