At the edge of the Milky Way, there are stars that move against the general flow, like cars driving in the wrong lane. Previously, it was thought that all of them were remnants of one dwarf galaxy that our Galaxy tore apart and swallowed. But new research shows that there were several such 'oncoming' galaxies.
The scientists selected stars with low orbital inclination and moderate elongation, using motion data from the Gaia telescope and chemical composition from spectra. Then they built a map where each star is marked by distance from the Galactic center and iron content. On this map, distinct groups emerged, like layers in a cake, each with its own recipe.
It turned out that groups with the same 'recipe' do not always belong to the same parent galaxy. For example, stars with an iron content of about -1.7 could have come from two different systems, absorbed at different times. The main contribution comes from one large galaxy, whose debris stretches into a continuous chain in energy and chemistry, like a string of beads being gradually pulled apart. But nearby there are also independent 'threads' from other, smaller galaxies.
Such a conclusion was only possible through a combined analysis of chemistry and dynamics. Separately, neither composition nor orbits give the full picture. It's like trying to reconstruct a city's history from a list of surnames without addresses — only by combining both lists do you see who came from where.
The wrong lane on the outskirts of our Galaxy turned out to be not a single road but an interweaving of several. And each of them leads to its own destroyed galaxy.
The Gaia telescope measures the positions and velocities of more than a billion stars with enough precision to notice a shift the thickness of a human hair from a distance of 1000 kilometers.