The merger that led to the formation of the Milky Way's inner stellar halo and thick disk (1806.06038v2)

Abstract: The assembly process of our Galaxy can be retrieved using the motions and chemistry of individual stars. Chemo-dynamical studies of the nearby halo have long hinted at the presence of multiple components such as streams, clumps, duality and correlations between the stars' chemical abundances and orbital parameters. More recently, the analysis of two large stellar surveys have revealed the presence of a well-populated chemical elemental abundance sequence, of two distinct sequences in the colour-magnitude diagram, and of a prominent slightly retrograde kinematic structure all in the nearby halo, which may trace an important accretion event experienced by the Galaxy. Here report an analysis of the kinematics, chemistry, age and spatial distribution of stars in a relatively large volume around the Sun that are mainly linked to two major Galactic components, the thick disk and the stellar halo. We demonstrate that the inner halo is dominated by debris from an object which at infall was slightly more massive than the Small Magellanic Cloud, and which we refer to as Gaia-Enceladus. The stars originating in Gaia-Enceladus cover nearly the full sky, their motions reveal the presence of streams and slightly retrograde and elongated trajectories. Hundreds of RR Lyrae stars and thirteen globular clusters following a consistent age-metallicity relation can be associated to Gaia-Enceladus on the basis of their orbits. With an estimated 4:1 mass-ratio, the merger with Gaia-Enceladus must have led to the dynamical heating of the precursor of the Galactic thick disk and therefore contributed to the formation of this component approximately 10 Gyr ago. These findings are in line with simulations of galaxy formation, which predict that the inner stellar halo should be dominated by debris from just a few massive progenitors.

• The paper confirms that a merger with Gaia-Enceladus significantly triggered the formation and dynamical heating of the Milky Way's thick disk.
• It employs chemo-dynamical analysis of Gaia DR2 data to distinguish stars with unique kinematics and chemical signatures from the Galaxy’s native population.
• The study supports hierarchical galaxy formation theories by linking an accreted progenitor comparable to the Small Magellanic Cloud to the present-day Galactic structure.

The Merger and Formation of the Milky Way's Inner Stellar Halo and Thick Disk

The paper by Helmi et al. presents a detailed chemo-dynamical analysis of the Milky Way's thick disk and inner stellar halo, focusing on an impactful merger event involving an external galaxy referred to as Gaia-Enceladus. This paper significantly enhances our understanding of the structure and formation history of our Galaxy.

Main Findings

The authors employ data from major stellar surveys, particularly the Gaia DR2, to paper kinematics, chemistry, age, and spatial distribution of stars surrounding the Sun. Their analysis confirms the presence of a prominent accreted structure, Gaia-Enceladus, which has had substantial implications for the development of the Galaxy. The proposed progenitor of this structure had a mass comparable to the Small Magellanic Cloud and significantly contributed to the formation and dynamical heating of the thick disk roughly 10 billion years ago. Various lines of evidence, including kinematic and chemical signatures, support this assertion:

• Kinematics: The velocity distribution within 2.5 kpc of the Sun reveals a significant number of stars associated with a slightly retrograde kinematic structure, distinct from the Galactic disk stars. The detailed morphology of this structure in velocity space is consistent with simulations of a thick disk formed through a merger with a satellite galaxy.
• Chemical Signatures: The distinct [$\alpha$/Fe] vs [Fe/H] abundance pattern of stars associated with Gaia-Enceladus, identified as a separate chemical sequence, distinguishes it from the thick disk. This necessitates a lower star formation rate for Gaia-Enceladus's progenitor compared to the Galaxy.
• Age-Metallicity Relationship: The globular clusters and RR Lyrae variables potentially originating from Gaia-Enceladus exhibit a consistent age-metallicity relation. These stars contribute to the halo's chemical diversity and indicate an ancient origin from a now-defunct galaxy.

Implications

The implications of this paper are profound both in practical and theoretical contexts:

• Galaxy Formation: The findings support the hierarchical formation model of galaxies, where significant components of the Galactic halo are remnants of a few massive progenitors. The merger with Gaia-Enceladus played an essential role in shaping the current structure of the Milky Way's thick disk, aligning with theoretical galaxy formation simulations.
• Kinematic and Chemical Tracers: The research underscores the efficiency of using advanced kinematic and chemical analyses as tools to unravel Galactic assembly history, which could be applied to other galaxies with similar datasets.

Future Directions

This paper opens up several pathways for future research:

• Expanded Volume Studies: Broader surveys stretching beyond the solar vicinity and utilizing future Gaia data releases could refine the understanding of the spatial extent and structure of Gaia-Enceladus.
• Refined Simulations: Coupling these findings with high-resolution simulations may offer more precise insights into the dynamical evolution and impact of such large-scale mergers on galaxy morphology.
• Chemical Evolution Models: Given the intricate chemical signatures left by Gaia-Enceladus, tighter integration with chemical evolution models may yield further understanding of stellar population diversity and star formation histories in the Galaxy.

In conclusion, this paper provides a comprehensive analysis of the impact of a significant merger event on the structure of the Milky Way's thick disk and inner stellar halo. It paves the way for future high-resolution investigations of galactic structure and formation history, illustrating the importance of detailed chemo-dynamical studies in advancing our understanding of galactic evolution.