Stellar Metallicities from SkyMapper Photometry II: Precise photometric
  metallicities of $\sim$280,000 giant stars with [Fe/H] $< -0.75$ in the Milky
  Way
  The Milky Way's metal-poor stars are nearby ancient objects that are used to
study early chemical evolution and the assembly and structure of the Milky Way.
Here we present reliable metallicities of $\sim280,000$ stars with $-3.75
\lesssim$ [Fe/H] $\lesssim -0.75$ down to $g=17$ derived using
metallicity-sensitive photometry from the second data release (DR2) of the
SkyMapper Southern Survey. We use the dependency of the flux through the
SkyMapper $v$ filter on the strength of the Ca II K absorption features, in
tandem with SkyMapper $u,g,i$ photometry, to derive photometric metallicities
for these stars. We find that metallicities derived in this way compare well to
metallicities derived in large-scale spectroscopic surveys, and use such
comparisons to calibrate and quantify systematics as a function of location,
reddening, and color. We find good agreement with metallicities from the
APOGEE, LAMOST, and GALAH surveys, based on a standard deviation of
$\sigma\sim0.25$dex of the residuals of our photometric metallicities with
respect to metallicities from those surveys. We also compare our derived
photometric metallicities to metallicities presented in a number of
high-resolution spectroscopic studies to validate the low metallicity end
([Fe/H] $< -2.5$) of our photometric metallicity determinations. In such
comparisons, we find the metallicities of stars with photometric [Fe/H] $<
-2.5$ in our catalog show no significant offset and a scatter of
$\sigma\sim$0.31dex level relative to those in high-resolution work when
considering the cooler stars ($g-i > 0.65$) in our sample. We also present an
expanded catalog containing photometric metallicities of $\sim720,000$ stars as
a data table for further exploration of the metal-poor Milky Way.
