Inside our galaxy they survive stellar clusters so old that they function as fossils in an archaeological record. Not all of them belong to the original Milky Way: according to research, some look like the remains of entire galaxies that were devoured billions of years ago. How much can we reconstruct of our galaxy’s infancy from these remains? A new analysis, published in the repository arXivproposes that one of those victims left an unmistakable signature. It’s called LKH and it says a lot about the past of our home in the cosmos.
How much can we reconstruct of our galaxy’s infancy from the stellar remains of the Milky Way?
Stellar fossils
The Globular clusters are very dense groupings of hundreds of thousands of stars that formed early in the galaxy’s history.. They’ve barely changed since then, as if no switch can reset their internal clock. That is why they behave like fossils: they retain the age, chemical composition and orbit they had when they were born or captured.
The key to reading them is metallicity, that is, the proportion of elements heavier than hydrogen and helium. A cluster formed within the Milky Way tends to show a characteristic relationship between its age and its metallicity. A cluster that arrives with an external galaxy usually deviates from that pattern. Thus, by adding movements and orbits, astronomers can distinguish globular clusters in our galaxy from those from other galaxies.
The study by Davide Massari and his team starts precisely from there. Its objective is not to observe a collision that occurred more than 12,000 million years ago, something physically impossible, but identify the trace that this event would have left on the fossils that we can still measure today.
Globular clusters behave like fossils: they retain the age, chemical composition and orbit they had when they were born or captured.
Three sequences that reveal three origins
The central result of the study indicates that globular clusters are not distributed randomly in the age-metallicity plane, but rather form three different sequences. One corresponds to the population born on sitewithin the Milky Way itself. Another fits Gaia–Enceladus, a well-known satellite galaxy, which was absorbed later. And a third would derive from a different progenitor, a dwarf galaxy that the study calls Low-energy–Kraken–Heracles (LKH)a name that unifies the identifications proposed in previous research works. This third sequence occupies an intermediate position. Despite its age, it is chemically distinguishable from both the original Milky Way and Gaia–Enceladus.
Observation and inference in the case of LKH
The research team bases its conclusions on three main levels of analysis. The first level derives from the observation. The team analyzed 39 globular clusters with dynamic information and homogeneous ages. Of them, 17 were dated again using photometry from the Hubble space telescope, using the F606W and F814W filters, and adjusting isochrones (the theoretical curves that relate brightness and color to the age of a stellar population) with the CARMA methodology. The average uncertainty of these 17 ages is around 0.26 gigayearsthat is, about 260 million years.
The second level concerns the simulation and modeling. The orbits were calculated within a barred galactic potential and magnitudes such as Jacobi energy, vertical action or orbital circularity were obtained. On these data, a Bayesian mixture model was applied, in which each possible progenitor contributes its own age-metallicity relationship. The study compared scenarios with two, three and four components. The scenario formed by three components (original Milky Way, Gaia–Enceladus and LKH) obtained the greatest statistical evidence, while the alternatives were clearly disfavored.
The third level is the interpretation. This is where the analysis attributes 12 of the 39 clusters to LKH by considering membership probable when the probability exceeds 50%. Therefore, this figure is not an unequivocal observational count, but rather an estimate within the model.
A galaxy of 500 million suns buried in the center
With these ingredients, the study proposes a concrete reconstruction. LKH would have been absorbed approximately 12.3 billion yearswhen the universe was around 1.5 billion years and the redshift was greater than 4. The progenitor galaxy would have contributed a stellar mass close to 500 million solar massescomparable to that of Gaia–Enceladus. Where were the remains of LKH integrated? Depending on the models used, the majority would be concentrated in the 6 interior kiloparsecs of the Milky Way, that is, some 19,600 light years around the galactic center.
The scenario formed by three components (original Milky Way, Gaia–Enceladus and LKH) obtained the greatest statistical evidence.
From LKH to Gaia–Enceladus: a galaxy that grew by bites
The interest in LKH grows when comparing it with what we already knew. In 2018, Amina Helmi and her team identified Gaia–Enceladus (also called Gaia’s Salsicha, due to the radial distribution of its stars) based on data from the Gaia mission. It is characterized by stars with very elongated orbits, a dominant radial motion and a low or slightly retrograde angular momentum. That satellite galaxy would have been incorporated a few years ago 10 billion years. It would have contributed to the inner stellar halo and would have heated a pre-existing disk, favoring the formation of the thick disk.
The chronology that emerges, therefore, is that of a galaxy that was not born suddenly. According to this reconstruction, the oldest populations would have formed first. Then, the possible merger with LKH would have occurred about 12.3 billion years ago, followed later by the incorporation of Gaia–Enceladus about 10 billion years ago. From there, the slow evolution of the halo and the album would have continued.
Where the evidence ends and the hypothesis begins
Given the very nature of the object of study, the research is based on statistical inferences, ages with margins of error and mixture models. Although the data suggests that the Milky Way experienced significant early merger, the details of that merger remain limited by measurements. It is still debated whether the three-component model truly captures the real history of our galaxy and whether each of the 12 clusters attributed to LKH actually belongs to that stellar population. All in all, we are a little closer to understanding how the place we inhabit in the cosmos was formed.
