Candidate Members of the VMP/EMP Disk System of the Galaxy from the SkyMapper and SAGES Surveys

Photometric stellar surveys now cover a large fraction of the sky, probe tofainter magnitudes than large-scale spectroscopic surveys, and are relativelyfree from the target-selection biases often associated with such studies.Photometric-metallicity estimates that include narrow/medium-band filters canachieve comparable accuracy and precision to existing low-resolutionspectroscopic surveys such as SDSS/SEGUE and LAMOST. Here we report on aneffort to identify likely members of the Galactic disk system among the verymetal-poor (VMP; [Fe/H] ≤ –2) and extremely metal-poor (EMP; [Fe/H]≤ –3) stars. Our analysis is based on an initial sample of ∼11.5million stars with full space motions selected from the SkyMapper SouthernSurvey (SMSS) and Stellar Abundance and Galactic Evolution Survey (SAGES).After applying a number of quality cuts to obtain the best availablemetallicity and dynamical estimates, we analyze a total of ∼5.86 millionstars in the combined SMSS/SAGES sample. We employ two techniques that,depending on the method, identify between 876 and 1,476 VMP stars (6.9of all VMP stars) and between 40 and 59 EMP stars (12.4stars) that appear to be members of the Galactic disk system on highly progradeorbits (v_ϕ > 150 km/s). The total number of candidate VMP/EMP disk-likestars is 1,496, the majority of which have low orbital eccentricities, ecc ≤0.4; many have ecc ≤ 0.2. The large fractions of VMP/EMP stars associatedwith the Milky Way disk system strongly suggest the presence of an earlyforming “primordial" disk.