The Pristine Survey – VIII. The metallicity distribution function of the Milky Way halo down to the extremely metal-poor regime

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.

TOPoS

“…In Fig. 13, we compare the normalised metal-weak tail of the MDF, as observed, for i) our whole sample; ii) the Hamburg/ESO sample (Schörck et al 2009); iii) the Pristine sample (Youakim et al 2020); and iv) the H3 sample (Naidu et al 2020). The H3 Survey adopts a target selection that is based only on apparent magnitudes, parallaxes, and Galactic latitude, thus they claim that their sample is unbiased with respect to metallicity.…”

“…In the last twenty years, there have been many attempts to define the MDF of the Galactic halo, a non-exhaustive list includes Schörck et al (2009), Yong et al (2013b), Allende Prieto et al (2014), Youakim et al (2020), Naidu et al (2020, and Carollo & Chiba (2020), and we compare our results with some of theirs. Our group has long been exploiting the spectra of the Sloan Digital Sky Survey (hereafter SDSS, York et al 2000;Adelman-McCarthy et al 2008;Ahn et al 2012;Alam et al 2015) to select extremely metal-poor stars (Caffau et al 2011;Bonifacio et al 2011;Caffau et al 2012; 2 Bond (1981) refers to any star with [Fe/H]< −3 as Population III.…”

“…A magnitude cut was applied to select stars with 19 < g 0 < 20 and colour 0.15 < (g − i) < 0.4, combined with the exclusion of a region in the (u − g) 0 versus (g − i) 0 plane that is dominated by young stars (see Fig. 3 of Youakim et al 2020). This sample amounts to about 80 000 stars.…”

“…The selection function for the four samples is clearly different. Schörck et al (2009) provide three possible bias corrections for their observed MDF, while Youakim et al (2020) provides two. In Fig.…”

Dynamical Simulations of the First Globular Clusters

“…The model has a single parameter, the effective enrichment yield, Y , of a generation of stars, and then predicts cumulative numbers N(< Z) ∝ 1 − exp (−Z/Y) with a normalization that depends on when star formation ceases, or, is matched to an observational data set (Hartwick 1976). Depending on the details of calibrating to Milky Way halo numbers, if globular clusters trace the field star metallicity distribution, then 3-10 clusters are expected below the apparent floor of [Fe/H] < −2.5 if the clusters continue to be in proportion to the numbers of field stars with metallicity (Youakim et al 2020).…”