The Pristine Survey – VIII. The metallicity distribution function of the Milky Way halo down to the extremely metal-poor regime. Issue 4 (16th January 2020)

• Record Type: Journal Article
• Authors: Youakim, K; Starkenburg, E; Martin, N F; Matijevič, G; Aguado, D S; Allende Prieto, C; Arentsen, A; Bonifacio, P; Carlberg, R G; González Hernández, J I; Hill, V; Kordopatis, G; Lardo, C; Navarro, J F; Jablonka, P; Sánchez Janssen, R; Sestito, F; Thomas, G F; Venn, K
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 492:Issue 4(2020)
• Page Start: 4986
• Page End: 5002
• Publication Date: 2020-01-16
• Subjects: stars: abundances -- Galaxy: abundances -- Galaxy: evolution -- Galaxy: formation -- Galaxy: halo -- galaxies: dwarf; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stz3619
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: The Pristine survey uses narrow-band photometry to derive precise metallicities down to the extremely metal-poor regime ($ \rm [Fe/H] \lt -3$ ), and currently consists of over 4 million FGK-type stars over a sky area of $\sim 2500\, \mathrm{deg}^2$ . We focus our analysis on a subsample of ∼80 000 main-sequence turn-off 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 $ \rm [Fe/H] =-1.6$, and a slope of Δ(LogN)/$\Delta \rm [Fe/H] = 1.0 \pm 0.1$ in the metallicity range of $-3.4\; \lt\; \rm [Fe/H]\; \lt -2.5$ . 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 anticentre direction, and also discuss the possibility that the inner halo is dominated by a single, large merger event, but cannot 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 $ \rm [Fe/H]\; \lt\; -2.5$ and 1.5 for $ \rm [Fe/H]\; \lt\; -3$, suggesting that the lack of low-metallicity globular clusters in the Milky Way is not due simply to statistical undersampling.