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.
as written by the authors arXiv:2604.13482
We investigate the progenitors of low-inclination retrograde substructures in the Milky Way (MW) halo, which are remnants of accreted dwarf galaxies on retrograde orbits. Our sample consists of halo stars with low orbital inclinations and eccentricities ($$0 \le e \le 0.5$$), constructed by combining spectroscopic data with $$\it Gaia$$ astrometry. We identify substructures using metallicity distribution functions (MDFs) in apogalactic distance-orbital phase space. In the low-eccentricity range ($$0 \le e \le 0.3$$), we find four substructures with MDF peaks at [Fe/H] $$\approx -1.5$$, $$-1.9$$, $$-2.1$$, and $$-2.3$$. In the intermediate-eccentricity range ($$0.3 < e \le 0.5$$), we identify five substructures that span [Fe/H] $$\approx -1.5$$ to $$-2.3$$. By combining chemical and dynamical information, we show that substructures with identical MDF peaks in the two eccentricity regions can either form coherent structures or remain dynamically distinct. This shows that MDF similarity alone is insufficient to uniquely identify progenitor systems and must be combined with dynamical information. We find that the retrograde halo was assembled through multiple accretion events rather than a single progenitor. The dominant contribution arises from a primary progenitor whose debris traces a coherent metallicity-energy sequence, consistent with hierarchical tidal stripping and core bifurcation. In addition, we identify independent progenitors that contribute to other substructures. In particular, the components with [Fe/H] $$\approx -1.7$$ are interpreted as a dual-origin population, likely associated with systems accreted at different epochs. These results highlight the complex, multi-progenitor origin of the retrograde stellar halo of the MW.
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.