Diana Carolina Zapata, the guest writer of today’s post, is a final-semester student in the Astronomy program at the University of Antioquia (Colombia). Diana conducted the research summarized below as part of the Red de Estudiantes Colombianos de Astronomía (RECA) Internship Program 2024 under the supervision of Dr. David Aguado, an astrophysicist at the Instituto de Astrofísica de Canarias (IAC). The preliminary results of this research were presented at the RECA Internship Symposium 2024.

Galactic mergers and the formation of the Milky Way

During its formation, the Milky Way experienced several collision events that shaped its current structure and dynamics. In particular, two major events stand out in the history of our galaxy. The first is known as Gaia-Sausage (or Enceladus). This event involved a collision with a dwarf galaxy that occurred approximately 10 billion years ago. This dwarf galaxy had a stellar mass of about 10% of the Milky Way’s mass, resulting in a highly energetic impact that reshaped our galaxy’s structure, influencing both its inner bulge and outer halo. The second event is known as Sequoia, which, although it represented a smaller collision compared to Gaia-Sausage, also left a significant mark on the evolutionary history of the Milky Way.

During these merger events, the progenitor galaxies of Sequoia and Gaia-Sausage disintegrated, leaving remnants that were incorporated into the Milky Way. Today, our understanding is that our galaxy has a halo largely composed of small systems that have been accreted over time, such as Gaia-Sausage and Sequoia.

A fascinating aspect of studying the galactic halo is the ability to identify stars that originated from Gaia-Sausage and Sequoia. These stars carry unique chemical signatures that allow us to reconstruct the history of the mergers that shaped the Milky Way. In this sense, they are like cosmic fossils, but with a crucial advantage: having merged with the Milky Way, these stars are relatively close to us. This proximity facilitates their detailed analysis through spectroscopy, taking advantage of the accessibility our instruments provide for their study.

The r-process and chemical evolution

In this research, we focused on studying the chemical abundances of these galactic halo stars, with special attention to elements formed through neutron capture processes. These processes occur when atomic nuclei capture neutrons in specific environments, either slowly (s-process) or rapidly (r-process). While potential production sites for r-process elements have been identified, it is still not fully understood which astrophysical events contribute the most to their total abundance in the Universe.

There are two main hypotheses regarding the origin of r-process elements, such as europium (Eu):

•  Neutron star mergers: Events in which two orbiting neutron stars lose gravitational energy until they merge in a violent explosion. 
• Type II supernovae: Explosive events that mark the end of the life cycle of very massive stars.

Since each nuclear production channel occurs under specific astrophysical conditions and is associated with different stellar events, incorporating a wide variety of chemical elements allows us to more comprehensively reconstruct the evolutionary history of the Milky Way. In this study, we analyze not only r-process elements but also other elements synthesized through different nuclear processes. This approach enables us to assess the chemical evolution of Gaia-Sausage and Sequoia with greater precision. By comparing different chemical abundances, we can infer whether the observed europium in these stars primarily originates from neutron star mergers (NSMs) or Type II supernovae, helping to shed light on the mystery of its origin.

Observational data and spectral analysis

Before this study, GyuChul Myeong identified seven stars associated with the remnants of Gaia-Sausage and Sequoia using the same method described in a previous work [1], based on Gaia data. Gaia is a space mission by the European Space Agency designed to map, with high precision, the position, motion, and other physical properties of over a billion stars in the Milky Way. Thanks to these data, it is possible to trace the galaxy’s dynamic history and past accretion events.

The high-resolution spectra of these stars were obtained using the FIES spectrograph, installed on the NOT telescope in the Canary Islands. An example of these spectra is shown in Figure 1, where some of the spectral lines of the chemical elements of particular interest for this research can be observed.

Figure 1. Normalized spectrum of one of the seven stars included in the study, highlighting the lines corresponding to the most relevant elements. In particular, europium (Eu) is emphasized as an r-process tracer, barium (Ba) as representative of the s-process, magnesium (Mg) as an alpha element, and iron (Fe) as a member of the iron-peak group.

Thanks to a detailed analysis of the spectral lines—including Gaussian fitting to precisely determine absorption intensities—the chemical abundances of the elements in all the stars were obtained. This allowed us to create various graphs illustrating the chemical evolution of Gaia-Sausage and Sequoia. The absorption intensity in a spectral line reflects the amount of a given element present in the stellar atmosphere, as specific atoms and ions absorb light at characteristic wavelengths. The stronger the absorption, the higher the abundance of the element in question.

Likewise, our results are presented alongside data from previous studies [1, 2], incorporating a wider range of metallicities and providing a more comprehensive view of the chemical evolution of these systems.

Key findings: Metallicity and chemical evolution

For the graphs that follow, it is important to keep in mind a key concept: metallicity can be interpreted as an indicator of time. At the beginning of the Universe, the first stars contained very few metals since there had been no previous stars to die and release heavy elements into the interstellar medium. As the Universe evolved and stars completed their life cycles, their explosions released new elements into space, gradually enriching the surrounding medium. This process led to a steady increase in metallicity in subsequent stellar generations.

For this reason, we can consider that at low metallicities, we are observing a more primitive Universe, whereas at higher metallicities, we see a more evolved environment. Thus, plotting data as a function of metallicity [Fe/H] helps us understand the chemical evolution of the systems we study, making this parameter particularly relevant to our analysis.

In Figure 2, in the left panel, the graph shows the relationship between metallicity [Fe/H] and [Eu/Fe]. For Gaia-Sausage, a significant dispersion in [Eu/Fe][Eu/Fe] is observed at low metallicities, which tends to stabilize as metallicity increases. Since [Eu/Fe] represents the amount of europium relative to iron, its evolution with respect to metallicity allows us to investigate how r-process elements were incorporated at different stages of stellar formation. The high dispersion at low metallicities suggests a delay in Eu production, indicating that some stars formed in environments with very little available europium, while others were born later, after the interstellar medium had already been enriched with this element.

One possible explanation is the production of Eu by NSMs (neutron star mergers), events that can take up to billions of years to occur, thereby extending the time interval over which the interstellar medium is enriched with this element.

In the right panel of Figure 2, the graph shows the ratio [Eu/Mg] versus [Mg/Fe], revealing an anticorrelation: in systems with lower [Mg/Fe], [Eu/Mg] tends to be higher. We know that magnesium is primarily produced in Type II supernovae. Therefore, if europium also originated predominantly from the same type of explosion, the [Eu/Mg] ratio should remain approximately constant. However, the observation of a phase where Eu production surpasses that of Mg suggests the contribution of another source—possibly NSMs—that enriches the interstellar medium with r-process elements.

Figure 2. The left panel illustrates the relationship between metallicity [Fe/H] and the relative abundance [Eu/Fe], allowing us to trace how europium was incorporated at different stages of stellar formation. The high dispersion observed at low metallicities suggests a delay in its production, supporting the hypothesis that r-process elements originate from events with long time delays, such as NSMs. The right panel shows the relationship between [Mg/Fe] and [Eu/Mg], where an anticorrelation is observed: at lower [Mg/Fe] values, [Eu/Mg] tends to be higher. Since magnesium is produced in Type II supernovae, if europium had the same origin, this ratio should remain constant. However, the fact that [Eu/Mg] increases at certain stages suggests the contribution of an additional source of r-process elements, such as NSMs. This analysis expands on previous results, providing evidence that NSMs likely play a crucial role in the chemical enrichment of the interstellar medium and the evolution of the Gaia-Sausage and Sequoia systems.

In Figure 3, using Mg instead of Fe reduces the influence of Type Ia supernovae, which are efficient at producing iron-peak elements but contribute less to magnesium production. This way, the graph primarily focuses on Type II supernovae, allowing us to study the evolution of [Eu/Mg]. The graph shows an increase in [Eu/Mg] and significant dispersion at low metallicities for Gaia-Sausage stars, which decreases as metallicity increases, leading to a tighter concentration of stars. This trend supports the existence of r-process sources with a significant delay, such as NSMs.

Figure 3. Relationship between [Mg/H] and [Eu/Mg], highlighting the contribution of Type II supernovae while minimizing the influence of Type Ia supernovae.

What we’ve learned and what’s next

A key aspect of this analysis was the comparison with data from the study by Ou et al. Xiaowei Ou (欧筱葳) kindly shared their results with us before publication, enriching our research and strengthening our interpretation. This comparison, along with data from Aguado (2021), validated the observed trends in europium enrichment and supports the hypothesis that neutron star mergers could be the primary source of r-process elements in the Universe, mainly due to their long time delays. 

However, the stars analyzed in our study do not cover a sufficiently wide range of metallicities to fully characterize the Sequoia system. An essential future step will be to obtain additional data on stars with even lower metallicities, allowing for a more comprehensive understanding of Sequoia’s chemical evolution and, ultimately, that of the Milky Way.

REFERENCES

1. Aguado, D. S., Belokurov, V., Myeong, G. C., et al. (2021). Elevated r-process Enrichment in Gaia Sausage and Sequoia.
2. Ou, X., Ji, A. P., Frebel, A., et al. (2024). The Rise of the R-Process in the Gaia-Sausage/Enceladus Dwarf Galaxy.