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Does the Milky Way contain less dark matter than previously thought?

The Milky Way rotation curve represents the circular rotational speed of stars as a function of distance to the Galactic center. The white dots and error bars represent the measurements obtained from the Gaia Data Release 3 catalogue. The blue curve represents the best adjustment of the rotation curve by a model including ordinary matter and dark matter. The yellow part of the curve shows the Keplerian decline with velocity V decreasing as R^-1/2, which begins beyond the optical disk of our Milky Way. It means that beyond the Galaxy's optical disk, its gravitational attraction is similar to that of a point mass. A constant rotation speed is rejected with a probability of 99.7% (3 sigma). Credits: Jiao, Hammer et al. / Observatoire de Paris – PSL / CNRS / ESA / Gaia / ESO / S. Brunier

Until now, the Milky Way was estimated to weigh about 10¹² solar masses. Part of this mass consists of the ordinary matter like stars and the cold gas in the Milky Way, expected to weigh about 0.6x10¹¹ solar masses. The rest of the mass of the Milky Way is made up of dark matter, a type of matter still unknown to us. The expected dark matter in the Milky Way was, until now, thought to be at least six times more than the ordinary matter and potentially even as much as 10 to 20 times the ordinary matter.

With new data come new insights, the estimates could be off. And not by a little.

The estimation of the Milky Way mass is heavily influenced by one of the assumptions made. The assumption that the Milky Way rotates in a certain way. Contrary to what is seen in for example the Solar System, where the orbital velocities of our planets are smaller when orbiting further away from the Sun (a velocity profile known as the Keplerian decline), in galaxies the observed rotation curve is that stars revolve around the galaxy centre at equal or even increasing speed when orbiting at larger distances from the centre compared to the ones orbiting closer to the centre.

However, now with the exquisite positions and 3D motions of stars in Gaia’s Data Release 3, the rotation curve in our own Milky Way galaxy can be derived like never before, as is done in the paper “Detection of the Keplerian decline in the Milky Way rotation curve”, published today in Astronomy & Astrophysics.

Using this newly derived rotation curve, a new mass was deduced for our Milky Way. And surprise... it is much lighter. It is now estimated to be only two hundred billion times that of the Sun (~2.06 x 10^11), so about four to five times lower than the previous estimates. The amount of ordinary matter did not change, thus, there must be a lot less dark matter in the Milky Way than previously thought. The new expectation is that the ordinary matter like stars and gas in the Milky Way now makes up about 1/3 of the Milky Way mass and the other 2/3 are accounted to dark matter. This finding could be a true revolution in cosmology.

Many questions remain. Why do we see a Keplerian decline in our own Milky Way, but not in the other galaxies observed. Does that mean our Milky Way is special in some way?

Measuring a rotation curve for a galaxy is not easy to achieve. For the many galaxies out there, the rotation curve is deduced from the observations of the neutral gas. For our own galaxy, and thanks to the Gaia space telescope, there is a high-quality six-dimensional data set for a large sample of stars, which can be used to compute the rotation curve. These are clearly different methodologies, and with this work, another step is taken in reassessing rotation curves for large spiral galaxies, and the amounts of ordinary versus dark matter inside.

Further reading:

• "Detection of the Keplerian decline in the Milky Way rotation curve" by Yongjun Jiao et al. (2023), published in Astronomy & Astrophysics, 27 September 2023
• Press release by the Observatoire de Paris - PSL

Story written by Tineke Roegiers

Credits: ESA/Gaia/DPAC, François Hammer, Observatoire de Paris

[Published: 27/09/2023]