Kavli Affiliate: Anna Frebel
| First 5 Authors: Mohammad K. Mardini, Anna Frebel, Anirudh Chiti, Yohai Meiron, Kaley V. Brauer
| Summary:
We have developed a chemo-dynamical 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 7,127 stars,
spans a metallicity range of $-3.50<${metal}~$<-0.8$, and has a systematic
rotational velocity of $langle V_phirangle=154$,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
$h_{Z}=1.68^{+0.19}_{-0.15}$,kpc. The metallicity distribution function is
well fit by an exponential with a slope of $Deltalog{rm
N}/Deltametal=1.13pm0.06$. Overall, we find a significant metal-poor
component consisting of 261 SkyMapper stars with metal$<-2.0$. While our
sample contains only eleven stars with {metal}~$lesssim-3.0$, investigating
the JINAbase compilation of metal-poor stars reveals another 18 such stars
(five have {metal}$<-4.0$) that kinematically belong to our sample. These
distinct spatial, kinematic and chemical characteristics strongly suggest this
metal-poor, phase-mixed kinematic sample to represent 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 as the Atari disk, given its likely accretion
origin, and in reference to it sharing space with the Galactic thin and thick
disks.
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