For years, scientists have traced the Milky Way’s growth through a series of cosmic collisions. The most recent is the ongoing assimilation of the Sagittarius dwarf galaxy, which began falling into our galactic plane about 6 billion years ago. Before that, the most massive known impact was the Gaia-Sausage-Enceladus (GSE) dwarf galaxy merger, which concluded roughly 10 billion years ago. That high-mass-ratio collision, estimated at 1:4 to 1:5, dynamically heated the galaxy and triggered the formation of the thick disk we observe today. Now, using exquisite data from the Hubble Space Telescope, researchers have looked even further back in time to uncover a previously hidden chapter in our cosmic history.
The Low-energy-Kraken-Heracles Merger
The newly identified progenitor system has been named Low-energy-Kraken-Heracles, or LKH. This massive galactic merger involved an object with a stellar mass similar to the Gaia-Sausage-Enceladus dwarf galaxy, approximately 5 × 10^8 solar masses. When it crashed into the infant Milky Way, LKH deposited most of its mass within the inner 6 kiloparsecs (kpc) of the galactic center.
Reconstructing the earliest phases of the Milky Way is notoriously difficult. As we look back more than 10 billion years, the mass of our galaxy was much smaller and more comparable to the systems it was absorbing. This makes it incredibly challenging to distinguish between stars born in the main progenitor (in situ) and those accreted from early building blocks. The inner galaxy is a chaotic environment where the rotating bar drives extremely short mixing timescales, effectively erasing much of the dynamical memory of ancient collisions. Furthermore, the intricacies of chemical evolution mean that state-of-the-art cosmological hydrodynamical simulations offer limited guidance on distinguishing these early building blocks from one another.
The identification of the LKH merger puts to rest long-standing debates about the origin of certain stellar populations in the inner galaxy. Previous attempts to isolate accreted stars from in situ populations yielded contested results, primarily due to cross-contamination. A group of globular clusters with low orbital energy, concentrated within 6 kpc of the galactic center, had been identified and linked to a theorized “Kraken” event. Similarly, a metal-poor, chemically unevolved population named “Heracles” was found, but its accreted origin was disputed because its chemical properties could also belong to early in situ populations. By establishing a precise timeline, researchers have confirmed that these populations indeed originated from a massive galactic merger, unifying the Kraken and Heracles hypotheses into the LKH progenitor.
Tracing History with Globular Clusters
The breakthrough relied on homogeneously analyzing a sample of 17 globular clusters in the inner Galaxy. Observed by the Hubble Space Telescope in the F606W and F814W optical bands, these clusters provided the data needed for highly precise relative age determination. By minimizing systematic errors that plagued previous heterogeneous datasets and applying a more sophisticated treatment of each cluster’s photometry, researchers achieved age estimates with typical errors of just a few hundred million years.
This analysis brought the total number of globular clusters with homogeneous age determinations to 39. The researchers applied a Bayesian model with multiple components, using the dynamic nested-sampling package dynesty, to determine the most likely number of progenitors required to describe the chrono-dynamical dataset. They assumed each progenitor followed an age-metallicity relation (AMR) described by a simple chemical evolution model.
The analysis revealed three distinct age-metallicity sequences. One sequence is associated with the Milky Way’s main progenitor, another with the Gaia-Sausage-Enceladus merger, and the third intermediate sequence corresponds to the LKH event. To map the dynamics, researchers computed the clusters’ Jacobi energy, average vertical action, and average circularity using a Galactic mass model that included a rotating bar, averaging the results over 200 orbits.
What Happens Next
The confirmation of the Low-energy-Kraken-Heracles merger fundamentally reshapes our understanding of early Milky Way formation. By providing a firm timeline for these early massive collisions, scientists can better constrain the cosmological frameworks that describe how large galaxies grow by consuming smaller systems. The method used in this study offers a template for untangling other complex galactic histories.
Future research will likely focus on identifying the specific stellar populations born during this ancient collision. As large spectroscopic surveys like APOGEE and GALAH continue to gather high-resolution data, astronomers will refine their chemical models to further separate in situ stars from accreted debris. The precise chronological sequencing provided by globular clusters will remain a crucial tool in this ongoing effort. As we look deeper into the inner galaxy, the chaotic remnants of the LKH merger will continue to yield secrets about the violent, foundational events that built our galactic home.
— Priya Nair, science desk, AXO News