Illustration par défaut

The mass of the Andromeda galaxy is revised downwards

3 February 2025 The mass of the Andromeda galaxy is revised downwards

Using the most sophisticated dynamical model of the Andromeda galaxy and of its past history, a team led by an astronomer from the LIRA at the Paris Observatory – PSL and CNRS has shown that its mass is much smaller than previously thought, implying a clear deficit of dark matter compared to the predictions of modern cosmology. The study is published in the journal Astronomy & Astrophysics, dated 3 February 2025.

Because it is our great neighbor, the Andromeda galaxy has played a key role in cosmology. In 1970, Vera Rubin and her colleagues discovered that the gas clouds on the edge of its disc were rotating too fast to be consistent with the mass of stars and gas of Andromeda. This demonstrated the need for a massive halo of dark matter surrounding this galaxy.

The scientific team of astronomers from the Paris Observatory – PSL, Universities of Aix-Marseille, Andres Bello of Santiago de Chile, Padua (Italy), and the Institut d’Astrophysique de Paris, has developed the most sophisticated dynamical model to determine the total mass of Andromeda. This model, made with the computational means of the Paris Observatory (MesoPSL), reproduces all the observations of this galaxy, from the central bar to the giant stream that dominates its halo. To reproduce all these properties and the rotation curve of Andromeda, astronomers have shown that its total mass is 450 billion solar masses.

In the past, the total mass of Andromeda was estimated to be two to four times higher than the new value. This drastic revision of the mass of Andromeda is mainly related to the history of this galaxy which suffered a gigantic collision 2.5 billion years ago. The published study shows that beyond a radius of 80,000 light-years, the orbiting material is no longer in balance with the gravitational forces of the galaxy because it has not had time to perform enough orbits since the great collision. This makes obsolete the large mass estimates for the Andromeda galaxy, which were based on astrophysical objects orbiting at distances between 100 000 and 1 000 000 light years.

The downwards revision of the mass of Andromeda immediately leads to a question to modern cosmology. For the latter, ordinary matter, made up of gas and stars, represents less than one sixth of the material content of the Universe. With the new measurement, it is a third of the mass of Andromeda that is made up of ordinary matter. This causes a dark matter deficiency in the Andromeda galaxy, conversely to what had been established for most large spiral galaxies with their flat rotation curves. In short, it lacks the missing mass in the galaxy where it was first identified. A contradiction that could challenge cosmological models.

Courbe de rotation de M31 superposée sur le disque de gaz qui montre la variation de la vitesse de rotation en fonction du rayon
Rotation curve of M31 superimposed on the gas disk, showing the variation in rotation speed as a function of radius.
The points correspond to the observed rotation curve of neutral hydrogen gas, which are perfectly reproduced by the motions of the simulated gas (magenta curve). They show a first “bump”, which represents the dynamical signature of the Andromeda disk. At the largest radii, there is a new growth in velocity, which is caused by gravitational instabilities induced by the collision that Andromeda suffered 2.5 billion years ago. The orange curve describes what this rotation curve would have been in the absence of gravitational instabilities, and represents the sum of the contributions of ordinary (green curve) and dark (gray curve) matter.
crédit : DR

Discover the origin of Andromeda’s formation on the Paris Observatory Youtube channel: link to the video

Reference

This work is published as an article entitled “Dark matter fraction derived from the M31 rotation curve”, by F. Hammer et al., to be published February the 3rd 2025 in Astronomy & Astrophysics (arXiv: 2412.02737).
https://www.aanda.org/10.1051/0004-6361/202452753

Collaboration

The team includes François Hammer (Observatoire de Paris – PSL), Yanbin Yang (Observatoire de Paris – PSL), Philippe Amram (Université d’Aix Marseille), Laurent Chemin (Universidad Andres Bello), Gary Mamon (Institut d’Astrophysique de Paris), Jianling Wang (Observatoire de Paris – PSL), Istiak Akib (Observatoire de Paris – PSL), Yongjun Jiao (Observatoire de Paris – PSL) and Haifeng Wang (Universita di Padova).

Know more

Scientific contact

  • François Hammer
    Astronome
    GEPI - Observatoire de Paris - PSL
    +33 (0) 1 45 07 74 08

Press contact

News