This study by Danny Horta and Ricardo P. Schiavon focuses on the early formation stages of the Milky Way by examining the oldest stars in our galaxy. By analyzing data from the APOGEE and Gaia surveys, the authors aim to shed light on the structure, density, and star formation history of the Milky Way’s proto-galactic components—essentially, the ancient fragments that contributed to forming the galaxy we observe today.

Introduction

The paper begins by emphasizing the importance of understanding how galaxies form, evolve, and merge over time. To do this, astronomers study both distant galaxies and the oldest stars in our Milky Way, referred to as the proto-Galaxy. These stars, which formed in the early Universe, retain chemical clues about the galaxy’s origin. The authors use a technique called "Galactic archaeology," where they use data from individual stars in the Milky Way to reconstruct its early history. In this study, they primarily focus on analyzing old stars to model the structure and mass of the Milky Way's earliest components.

Data and Methods

Horta and Schiavon rely on data from two major surveys: APOGEE, which provides detailed chemical compositions of stars, and Gaia, which tracks their positions and motions. By combining this information, the researchers can map out the structure and behavior of these ancient stars. Specifically, they look at stars that show distinct chemical signatures, particularly focusing on magnesium (Mg), aluminum (Al), manganese (Mn), and iron (Fe). These elements help differentiate between stars that formed in different parts of the early Milky Way and those that originated in other galaxies that later merged with ours.

Dissecting the Proto-Galaxy Chemically

In one of the key sections, the authors explore how the chemical compositions of these stars vary. They use a graph known as the [Mg/Mn] vs. [Al/Fe] plane to classify stars into two main groups: those that formed rapidly in the early Milky Way and those from slower-forming systems, likely brought in through galactic mergers. The stars from the first group likely belonged to the Milky Way's "main progenitor," while the second group includes stars from the Gaia-Enceladus-Sausage event, a major galactic collision in the Milky Way’s past. These distinctions help the authors model the early Milky Way’s growth.

Structural Modeling of the Proto-Galaxy

The authors go further by modeling the density and distribution of these ancient stars. Their findings suggest that the Milky Way’s proto-galaxy can be described using a "Plummer model," which is often used to describe the distribution of stars in elliptical galaxies. This model fits the data well and indicates that the proto-galaxy likely had a relatively low mass compared to the rest of the Milky Way, with a dense, flattened core concentrated in the inner regions of the galaxy.

Mass and Density Estimates

Horta and Schiavon estimate the mass of the Milky Way's proto-galaxy by integrating their density model for stars with specific chemical signatures. They calculate that the proto-galaxy has a stellar mass of about 9.1 x 108 solar masses. This provides new insights into the early structure and composition of the Milky Way, suggesting that the proto-galaxy was a relatively low-mass system with a metal-poor core.

Conclusion

The study concludes that the Milky Way’s proto-galaxy was composed of at least two major components: a "main progenitor" and the remains of the Heracles structure, a likely fragment of a galaxy that merged with the Milky Way early in its history. These findings provide a deeper understanding of the Milky Way’s early formation and offer a method for studying other galaxies’ growth over time.

Source: Horta and Schiavon