07 October 2021M31 and Milky Way encounter

© 2021 Observatoire Astronomique de Strasbourg | Webdesign et développement Alchimy.

Andromeda galaxy and Milky Way encounter becomes clearer thanks to latest Gaia data

Will we see the Andromeda galaxy and the Milky Way merge in a few billion years? This is the question that the results of this study involving researchers from the Besançon and Strasbourg observatories will help to answer.

Astrometric data published in the latest catalogue of the Gaia mission (EDR3) in December 2020 have enabled a team of researchers (including Benoit Famaey and Rodrigo Ibata from the Strasbourg Astronomical Observatory) to obtain the proper motion of the Andromeda galaxy (M31) with unprecedented accuracy.

The Milky Way and M31 are giant spiral galaxies of similar mass. This pair gravitationally dominates the Local Group – our immediate galactic environment that includes dozens of other smaller galaxies with different morphologies such as the Magellanic Clouds or the Triangulum Galaxy. Such an arrangement of our neighbourhood makes it very interesting to study the dynamics of this group. The velocity of Andromeda relative to the Milky Way can indeed give us information on the formation of the Local Group, and thus of our Galaxy. This velocity also gives us the keys to a better understanding of the impact of the galactic environment in the evolution of the Milky Way: did Andromeda play a role? If so, which one? Finally, the relative velocity between the Milky Way and Andromeda obviously sheds light on the future of the vault of heaven.

The radial velocity – on the line of sight – of the Andromeda galaxy can be obtained very precisely by spectrometry using the Doppler effect. It has been known for more than a century that it is approaching the Milky Way at about 104 km/s. The transverse velocity, on the other hand – the speed at which it moves across the sky or its proper motion – is much more difficult to obtain because it requires very precise astrometric observations. Therefore, while Andromeda’s proximity and similarity to our Galaxy make it a prime object of study, its transverse velocity has been poorly constrained until now.



Density on the sky of the positions of objects in the Gaia EDR3 catalogue in the direction of the Andromeda galaxy. Each orange point represents the position of an object in the catalogue in equatorial coordinates, the blue points are the positions of the stars identified as belonging to M31 and used in our study. The light blue arrow shows the proper motion of M31 with respect to the Milky Way as derived from this study. It will take about 80 million years for the position of the centre of the galaxy to move by the length of the blue arrow. For comparison, the black arrows show, to scale, the rotation velocity of the galaxy projected onto the sky at different locations. The dotted ellipse delimits the spatial extent of the galaxy considered in this study. Credits: Jean-Baptiste Salomon.

This value was recently derived using the proper motion of 1919 stars identified as belonging to the Andromeda disc and having good astrometric measurements. After modelling the rotational motion of the disc, as well as the observational and methodological biases, the overall motion of these stars was determined. The result is a relative transverse velocity between M31 and the Milky Way of 42 ± 39 km/s to the east and 59 ± 30 km/s to the south.

While the precision of these new estimations is unprecedented, the still significant uncertainties do not yet allow us to prejudge a date for the possible encounter between Andromeda and the Milky Way. However, it is now almost certain that the two galaxies are gravitationally linked and therefore have a past and a future in common.

Cover image : Image of the Andromeda galaxy (M31) in visible light. Digitized Sky Survey – STScI/NASA, color composition CDS, Observatoire astronomique de Strasbourg, CNRS/Unistra

Publication : “The proper motion of Andromeda from Gaia EDR3: confirming a nearly radial orbit“, Monthly Notices of the Royal Astronomical Society, Volume 507, Issue 2, October 2021

Researchers : Benoit Famaey, Rodrigo Ibata