Astronomer studies cosmic fossils to trace Milky Way's formation

Scientists have long known that the Milky Way did not form in isolation; it is the product of countless mergers and collisions that occurred over the past ten billion years. Galactic archaeologist Amina Helma, a researcher at the Institute for Astrophysics in Heidelberg, has devoted her career to locating and studying the remnants of these ancient interactions—so‑called “cosmic fossils.” By mapping streams of stars, globular clusters, and interstellar debris that retain the chemical fingerprints of their progenitor galaxies, Helma and her team reconstruct the sequence of accretion events that built the present‑day disk, bulge, and halo of our galaxy.

Helma’s latest survey, conducted with the European Southern Observatory’s VISTA telescope and the Gaia space observatory, has identified several previously unknown stellar streams that trace back to dwarf galaxies that were cannibalized by the Milky Way between 8 and 12 billion years ago. Spectroscopic analysis of these streams reveals distinct metallicity patterns and age distributions, allowing astronomers to date the mergers with unprecedented precision. The data also provide constraints on the mass growth of the Milky Way’s dark matter halo, offering insights into how such massive structures evolve over cosmic time and how future accretion events might reshape the galaxy’s architecture.

By piecing together the Milky Way’s violent past, Helma’s work not only illuminates the processes that forged our galactic home but also informs predictions about its future. The patterns of stellar motion and chemical composition uncovered in her studies suggest that the Milky Way will continue to absorb smaller satellite galaxies, gradually thickening its disk and enriching its stellar populations. As the field of galactic archaeology advances, these “cosmic fossils” will remain key to understanding the lifecycle of galaxies in the universe.