The Metal-Poor Metallicity Distribution of the Ancient Milky Way

Abstract

We present a low metallicity map of the Milky Way consisting of $\sim$111,000 giants with $-3.5\lesssim$ [Fe/H] $\lesssim -$0.75, based on public photometry from the second data release of the SkyMapper survey. These stars extend out to $\sim$7kpc from the solar neighborhood and cover the main Galactic stellar populations, including the thick disk and the inner halo. Notably, this map can reliably differentiate metallicities down to [Fe/H] $\sim -3.0$, and thus provides an unprecedented view into the ancient, metal-poor Milky Way. Among the more metal-rich stars in our sample ([Fe/H] $>-2.0$), we recover a clear spatial dependence of decreasing mean metallicity as a function of scale height that maps onto the thick disk component of the Milky Way. When only considering the very metal-poor stars in our sample ([Fe/H] $<-$2), we recover no such spatial dependence in their mean metallicity out to a scale height of $|Z|\sim 7$ kpc. We find that the metallicity distribution function (MDF) of the most metal-poor stars in our sample ($-3.0<$ [Fe/H] $<-2.3$) is well fit with an exponential profile with a slope of $\Delta \log (N)/\Delta$[Fe/H] = 1.52$\pm$0.05, and shifts to $\Delta \log (N)/\Delta$[Fe/H] = 1.53$\pm$0.10 after accounting for target selection effects. For [Fe/H] $<-2.3$, the MDF is largely insensitive to scale height $|Z|$ out to $\sim 5$kpc, showing that very and extremely metal-poor stars are in every galactic component.

9 pages, 5 figures; accepted for publication in ApJL. Minor corrections after acceptance addressing referee report for Chiti et al. ApJS submitted

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