The Biggest Splash (1909.04679v1)

Abstract: Using a large sample of bright nearby stars with accurate Gaia Data Release 2 astrometry and auxiliary spectroscopy we map out the properties of the principle Galactic components such as the "thin" and "thick" discs and the halo. We show that in the Solar neighborhood, there exists a large population of metal-rich ([Fe/H]>-0.7) stars on highly eccentric orbits. By studying the evolution of elemental abundances, kinematics and stellar ages in the plane of azimuthal velocity v_phi and metallicity [Fe/H], we demonstrate that this metal-rich halo-like component, which we dub the Splash, is linked to the alpha-rich (or "thick") disc. Splash stars have little to no angular momentum and many are on retrograde orbits. They are predominantly old, but not as old as the stars deposited into the Milky Way in the last major merger. We argue, in agreement with several recent studies, that the Splash stars may have been born in the Milky Way's proto-disc prior to the massive ancient accretion event which drastically altered their orbits. We can not, however, rule out other (alternative) formation channels. Taking advantage of the causal connection between the merger and the Splash, we put constraints of the epoch of the last massive accretion event to have finished 9.5 Gyr ago. The link between the local metal-rich and metal-poor retrograde stars is confirmed using a large suite of cutting-edge numerical simulations of the Milky Way's formation.

• The paper identifies the Splash as a unique group of metal-rich stars on eccentric, often retrograde orbits, suggesting an in-situ origin over merger accretion.
• Using Gaia DR2 and extensive spectroscopic data, the study reveals that the Splash likely formed from the proto-disc heated by a significant merger about 9.5 billion years ago.
• Comparisons with cosmological simulations and spatial analyses from SDSS and LAMOST underscore the Splash's role in advancing our understanding of the Milky Way’s formation and evolution.

Overview of "The Biggest Splash" by Belokurov et al.

This paper presents an in-depth analysis of the Milky Way’s stellar components using an extensive dataset derived from Gaia Data Release 2 (DR2) and extensive spectroscopic surveys. The paper particularly focuses on an enigmatic population of metal-rich stars ([Fe/H] > -0.7) on eccentric orbits in the solar neighborhood, which the authors refer to as the "Splash."

The paper identifies the Splash as a distinct element within the broader structure of the Galaxy, which includes the thin and thick discs and the stellar halo. The Splash is differentiated from these components by its unique kinematic and chemical properties. Specifically, it is shown that the Splash stars exhibit little to zero angular momentum and include many with retrograde orbits—a contrast to past theories that associated similar stars with accreted material from a merger event, the so-called Gaia Sausage.

Key Findings

1. Presence of Metal-rich Stars on Eccentric Orbits: The paper identifies a significant group of metal-rich stars, distinct in their kinematic behavior, suggesting an in-situ rather than an accreted origin. This challenges previous assumptions that all such stars in the halo originate from ancient merger events.
2. Formation Scenario: The authors propose that the Splash likely originated from the Milky Way’s proto-disc, which was heated by the last significant merger that the Galaxy experienced approximately 9.5 billion years ago. This interaction transformed a portion of the disc population into the halo-like orbits observed today.
3. Spatial Distribution and Comparison with Models: By examining the spatial extent using K-giant star data from SDSS and LAMOST, the paper reveals that the Splash is more confined than other halo components, supporting the hypothesis of an in-situ origin. Comparisons with zoom-in cosmological simulations (e.g., Auriga and Latte) corroborate the observational data, illustrating how such features might arise from galaxy formation processes.
4. Implications for Galactic Assembly: The paper provides critical insights into the understanding of the Milky Way’s formation and evolution. The Splash represents a historical record of a formative period in the Galaxy's history, likely linked to a massive accretive event. It influences models of galactic formation, suggesting that galaxies can retain dense, dynamically hot stellar populations even amidst the upheaval of major mergers.

Implications and Future Directions

The findings underscore the complexity of the Milky Way's structural formation, highlighting the need for models that account for both in-situ star formation and the dynamical response of stellar populations to merger events. This paper also emphasizes the importance of kinematic surveys in unraveling galactic histories. The ability to spatially resolve the Splash speaks to the richer story of the Galaxy’s development—from a rotation-supported proto-disc to the complex present-day structure seen in the Milky Way.

Further research is warranted to explore the chemical abundance distributions across different stellar populations within the Galaxy. Additionally, ongoing and upcoming missions that provide higher precision astrometric and spectroscopic data will enable a more refined analysis of these distinct galactic components, potentially uncovering further details about the merger history and evolution of our Galaxy.