The Metal-poor Metallicity Distribution of the Ancient Milky Way

We present a low-metallicity map of the Milky Way consisting of ∼110,000 metal-poor giants with −3.5 < [Fe/H] < −0.75, based on public photometry from the second data release of the SkyMapper survey. These stars extend out to ∼7 kpc 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] ∼ −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∣ ∼ 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 and ${\rm{\Delta }}\mathrm{log}(N)/{\rm{\Delta }}$[Fe/H] = 1.52 ± 0.05, and slightly shifts to ${\rm{\Delta }}\mathrm{log}(N)/{\rm{\Delta }}[\mathrm{Fe}/{\rm{H}}]=1.53\pm 0.10$ after accounting for target selection effects. For [Fe/H] < −2.3, the MDF is largely insensitive to scale height ∣Z∣ out to ∼5 kpc, showing that very and extremely metal-poor stars are in every galactic component.