A Blueprint for the Milky Way's Stellar Populations: The Power of Large
  Photometric and Astrometric Surveys
  Recent advances from astronomical surveys have revealed spatial, chemical,
and kinematical inhomogeneities in the inner region of the stellar halo of the
Milky Way Galaxy. In particular, large spectroscopic surveys, combined with
Gaia astrometric data, have provided powerful tools for analyzing the detailed
abundances and accurate kinematics for individual stars. Despite these
noteworthy efforts, however, spectroscopic samples are typically limited by the
numbers of stars considered; their analysis and interpretation are also
hampered by the complex selection functions that are often employed. Here we
present a powerful alternative approach $-$ a synoptic view of the spatial,
chemical, and kinematical distributions of stars in the Milky Way based on
large photometric survey databases, enabled by a well-calibrated technique for
obtaining individual stellar metal abundances from broad-band photometry. We
combine metallicities with accurate proper motions from the Gaia mission along
the Prime Meridian of the Galaxy, and find that various stellar components are
clearly separated from each other in the metallicity versus rotation-velocity
space. The observed metallicity distribution of the inner-halo stars deviates
from the traditional single-peaked distribution, and exhibits complex
substructures comprising varying contributions from individual stellar
populations, sometimes with striking double peaks at low metallicities. The
substructures revealed from our less-biased, comprehensive maps demonstrate the
clear advantages of this approach, which can be built upon by future mixed-band
and broad-band photometric surveys, and used as a blueprint for identifying the
stars of greatest interest for upcoming spectroscopic studies.
