Astronomers know that galaxies grow larger by merging with other galaxies over time. We see this happening in our galaxy. The Milky Way is slowly swallowing the Large and Small Magellanic Clouds and the Sagittarius Dwarf Spherical Galaxy.

For the first time, astronomers have found evidence of an ancient mass migration of stars to another galaxy. They detected more than 7,000 stars in our nearest neighbor, Andromeda (M31), which merged into a galaxy about two billion years ago.

The growth and evolution of galaxies is a hot topic in astronomy and is one of the reasons why the James Webb Space Telescope has been making many headlines lately. One of the main scientific goals of JWST is to look back at the universe’s oldest galaxies to understand how they grew and evolved into what they are today. But this is not the only telescope that can shed light on this problem.

“Galaxy like M31 and our Milky Way have been built from the building blocks of many small galaxies throughout cosmic history.” Arjun Dey, NOIRLab

These new observations of Andromeda and the inward migration of stars come from the Dark Energy Spectroscopic Instrument (DESI). It was created to measure the effect of dark energy on the expansion of the universe. This is done by collecting the optical spectra of tens of millions of objects, most of them galaxies and quasars, and then creating a 3D map of the results.

DESI is similar to the better known Gaia spacecraft. Gaia has the ambitious goal of accurately mapping the position and motion of billions of stars in the Milky Way. Gaia data has led to many discoveries about our own galaxy. However, this is limited to mapping stars in the Milky Way.

Now, thanks to DESI, astronomers have for the first time at least a partial map of Andromeda’s stars. And this map, which includes the movement of some 7,500 stars in the inner halo of the Andromeda galaxy, reveals their history.