The Pristine survey uses narrow-band photometry to derive precise metallicities down to the extremely metal-poor regime ( [ Fe / H ] < -3 ) , and currently consists of over 4 million FGK-type stars over a sky area of \sim 2 ~ { } 500 \mathrm { deg } ^ { 2 } .
We focus our analysis on a subsample of \sim 80 ~ { } 000 main sequence turnoff stars with heliocentric distances between 6 and 20 kpc , which we take to be a representative sample of the inner halo .
The resulting metallicity distribution function ( MDF ) has a peak at [ Fe / H ] = -1.6 , and a slope of \Delta ( LogN ) / \Delta [ Fe / H ] = 1.0 \pm 0.1 in the metallicity range of -3.4 < [ Fe / H ] < -2.5 .
This agrees well with a simple closed-box chemical enrichment model in this range , but is shallower than previous spectroscopic MDFs presented in the literature , suggesting that there may be a larger proportion of metal-poor stars in the inner halo than previously reported .
We identify the Monoceros/TriAnd/ACS/EBS/A13 structure in metallicity space in a low latitude field in the anticenter direction , and also discuss the possibility that the inner halo is dominated by a single , large merger event , but can not strongly support or refute this idea with the current data .
Finally , based on the MDF of field stars , we estimate the number of expected metal-poor globular clusters in the Milky Way halo to be 5.4 for [ Fe / H ] < -2.5 and 1.5 for [ Fe / H ] < -3 , suggesting that the lack of low metallicity globular clusters in the Milky Way is not due simply to statistical undersampling .