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Understanding local stellar kinematic substructures in the solar neighbourhood helps build a complete picture of the formation of the Milky Way, as well as an empirical phase space distribution of dark matter that would inform detection experiments. We apply the clustering algorithm hdbscan on the Gaia early third data release to identify a list of stable clusters in velocity space and action-angle space by taking into account the measurement uncertainties and studying the stability of the clustering results. We find 1405 (497) stars in 23 (6) robust clusters in velocity space (action-angle space) that are consistently not associated with noise. We discuss the kinematic properties of these structures and study whether many of the small clusters belong to a similar larger cluster based on their chemical abundances. They are attributed to the known structures: the Gaia Sausage-Enceladus, the Helmi Stream, and globular cluster NGC 3201 are found in both spaces, while NGC 104 and the thick disc (Sequoia) are identified in velocity space (action-angle space). Although we do not identify any new structures, we find that the hdbscan member selection of already known structures is unstable to input kinematics of the stars when resampled within their uncertainties. We thereforemore »present the stable subset of local kinematic structures, which are consistently identified by the clustering algorithm, and emphasize the need to take into account error propagation during both the manual and automated identification of stellar structures, both for existing ones as well as future discoveries.

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Abstract We have developed a chemodynamical approach to assign 36,010 metal-poor SkyMapper stars to various Galactic stellar populations. Using two independent techniques (velocity and action space behavior), Gaia EDR3 astrometry, and photometric metallicities, we selected stars with the characteristics of the “metal-weak” thick-disk population by minimizing contamination by the canonical thick disk or other Galactic structures. This sample comprises 7127 stars, spans a metallicity range of −3.50 < [Fe/H] < −0.8, and has a systematic rotational velocity of 〈 V ϕ 〉 = 154 km s −1 that lags that of the thick disk. Orbital eccentricities have intermediate values between typical thick-disk and halo values. The scale length is h R = 2.48 − 0.05 + 0.05 kpc, and the scale height is h Z = 1.68 − 0.15 + 0.19 kpc. The metallicity distribution function is well fit by an exponential with a slope of Δ log N / Δ [ Fe / H ] = 1.13 ± 0.06 . Overall, we find a significant metal-poor component consisting of 261 SkyMapper stars with [Fe/H] < −2.0. While our sample contains only 11 stars with [Fe/H] ≲ −3.0, investigating the JINAbase compilation of metal-poor stars reveals another 18 such starsmore »(five have [Fe/H] < −4.0) that kinematically belong to our sample. These distinct spatial, kinematic, and chemical characteristics strongly suggest that this metal-poor, phase-mixed kinematic sample represents an independent disk component with an accretion origin in which a massive dwarf galaxy radially plunged into the early Galactic disk. Going forward, we propose to call the metal-weak thick-disk population the Atari disk, given its likely accretion origin, and in reference to it sharing space with the Galactic thin and thick disks.« less