The birth of the Milky Way as uncovered by accurate stellar ages with Gaia

Abstract: Knowledge of ages for stars formed over a galaxy's lifetime is fundamental to understand its formation and evolution. However, stellar ages are difficult to obtain since they cannot be measured from observations, being comparison with stellar models (Soderblom 2010) required. Alternatively, age distributions can be derived applying the robust technique of colour-magnitude diagram fitting (Gallart et al. 2005), till now mainly employed to study nearby galaxies. The new distances to individual Milky Way stars from the Gaia mission (Brown et al. 2018) have allowed us to use this technique to derive ages from a thick disk colour-magnitude diagram, and from the enigmatic, two-sequenced colour-magnitude diagram of the kinematically hot local halo (Babusiaux et al. 2018), which blue-sequence has been linked to a major accretion event (Haywood et al. 2018, Helmi et al. 2018). Because accurate ages were lacking, the time of the merger and its role on our Galaxy's early evolution remained unclear. We show that the stars in both halo sequences share identical age distributions, and are older than the bulk of thick disc stars. The sharp halo age cut 10 Gyr ago can be identified with the accretion of Gaia-Enceladus. Along with state-of-the-art cosmological simulations of galaxy formation (Brook et al. 2012), these robust ages allow us to order the early sequence of events that shaped our Galaxy, identifying the red-sequence as the first stars formed within the Milky Way progenitor which, because of their kinematics, can be described as its long sought in-situ halo.

Overview of "The Birth of the Milky Way as Uncovered by Accurate Stellar Ages with Gaia"

The paper "The Birth of the Milky Way as Uncovered by Accurate Stellar Ages with Gaia" provides a detailed analysis of the formation and evolution of the Milky Way Galaxy using accurately determined stellar ages derived from data provided by the Gaia mission. This research leverages Gaia's second data release, which includes precise measurements of distances and luminosities, to construct color-magnitude diagrams (CMDs) that span diverse structural components within the Galaxy, such as the thick disk and the stellar halo.

Key Findings

The authors focused on constructing CMDs using absolute magnitudes and colors, which allowed them to derive the star formation histories and age distributions through comparison with theoretical CMDs built from stellar evolution models. The CMD of the kinematically defined stellar halo, containing two distinct sequences—referred to as the blue and red sequences—was of particular interest. The blue sequence is attributed to the Gaia-Enceladus accretion event, a significant merger that was part of the Galaxy's formation process.

• Age Distributions: The study finds that both sequences within the halo share similar age distributions, with ages older than the majority of stars in the thick disk. This coeval nature points to a common formation time, tracing back to the early Universe.

• Merger Identification: A sharp age cutoff for the halo stars around 10 billion years ago correlates with the accretion of Gaia-Enceladus. This event significantly influenced the early development of the Milky Way.

• Metallicity Insight: There is a notable distinction between the metallicities of the stars in the red and blue sequences. This suggests that the stars in the red sequence, being more metal-rich, originated from a more massive galaxy than those in the blue sequence. This metallicity difference supports the interpretation of a merger event where the red sequence stars belonged to the main progenitor of the Milky Way, whereas the blue sequence stars were part of Gaia-Enceladus.

Implications and Future Directions

This research provides crucial insights into the early Galactic events and their long-term effects on the Milky Way's structure. The accurate stellar ages derived mark a significant advancement over previous estimates, often lacking robust distance and luminosity data, thus leading to varied conclusions about Galactic evolution.

• Theoretical Implications: The coeval nature and metallicity differences outlined could refine our understanding of galaxy formation models, particularly concerning mergers involving massive and less massive galaxy counterparts.

• Practical Implications: Such detailed stellar age distributions and metallicity analyses could foster more accurate predictions in cosmological simulations, enhancing our understanding of not only the Milky Way but also the processes governing other similar galaxies.

Looking forward, the findings encourage further investigation using Gaia data to explore other Galactic components and structures, potentially unraveling more past merger events and refining models of star formation history. Enhanced simulations incorporating such empirical evidence could reshape our theories of galaxy evolution, applying these insights on a broader cosmological scale.

In conclusion, the paper significantly contributes to our understanding of the Milky Way by providing a detailed chronology of its formative events, using cutting-edge data from Gaia to map out the timelines and dynamics of stellar population formation in our Galaxy.