2022ApJ...936...78M

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 ${\rm{\Delta }}\mathrm{log}N/{\rm{\Delta }}[\mathrm{Fe}/{\rm{H}}]=1.13\,\pm \,0.06$. 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 stars (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.