We obtain a new determination of the metallicity distribution function ( MDF ) of stars within \sim 5 – 10 kpc of the Sun , based on recently improved co-adds of ugriz photometry for Stripe 82 from the Sloan Digital Sky Survey .
Our new estimate uses the methodology developed previously by An et al . to study in situ halo stars , but is based on a factor of two larger sample than available before , with much-improved photometric errors and zero-points .
The newly obtained MDF can be divided into multiple populations of halo stars , with peak metallicities at [ Fe/H ] \approx - 1.4 and -1.9 , which we associate with the inner-halo and outer-halo populations of the Milky Way , respectively .
We find that the kinematics of these stars ( based on proper-motion measurements at high Galactic latitude ) supports the proposed dichotomy of the halo , as stars with retrograde motions in the rest frame of the Galaxy are generally more metal-poor than stars with prograde motions , consistent with previous claims .
In addition , we generate mock catalogs of stars from a simulated Milk Way halo system , and demonstrate for the first time that the chemically- and kinematically-distinct properties of the inner- and outer-halo populations are qualitatively in agreement with our observations .
The decomposition of the observed MDF and our comparison with the mock catalog results suggest that the outer-halo population contributes on the order of \sim 35 \% – 55 \% of halo stars in the local volume .