Author: Kon
Reviewer: Nancy

  Modern cosmology holds that galaxies undergo many mergers between their initial formation and the present day. The details of the process are far too complicated for a complete analytical treatment, so the most common research method is to run direct numerical simulations of large numbers of point particles. Astronomy, of course, is ultimately an observational science. Researchers measure the properties of galaxies at different distances—the farther away a galaxy is, the older the universe and the younger the galaxy we see—and compare those measurements with numerical simulations to assess how well a theoretical model works.

A numerical simulation of the universe's large-scale structure produced by the Max Planck Institute in Germany. The most highly magnified region is a large galaxy cluster, where each small point of light represents a galaxy.

  Reconstructing the evolutionary history of galaxies has long been an important problem in astronomy. Because we live inside the Milky Way, our own galaxy is an exceptionally useful object of study. Tracing its history provides a direct test of current theories. Advances in observational technology have now made that work possible, and “galactic archaeology” has naturally become a major field in the era of astronomical big data.

  Galactic archaeology uses the chemical and dynamical properties of stars in the Milky Way to identify unusual structures such as stellar streams and tidal debris. Stars carry a kind of memory. The environments in which old stars formed were very different from the conditions around them today, as were the abundances of different heavy elements. Those signatures can still be read in the atmospheres of low-mass stars with long main-sequence lifetimes. Stars also preserve aspects of their original dynamics. If a population of stars was accreted from another galaxy during some stage of the Milky Way’s history, its motion may differ sharply from that of the Galaxy’s native population while remaining highly coherent within the group. Substructures with distinctive chemical and dynamical signatures therefore survive much like fossil evidence.

  Astronomers have now found many such substructures within the Milky Way. Some appear to be the remains of globular clusters torn apart by tidal forces; others are traces left by past mergers between the Milky Way and dwarf galaxies.

The spatial distribution of currently known substructures in the Milky Way. Source: arXiv:2002.04340.