The Metal-Poor Metallicity Distribution of the Ancient Milky Way

Kavli Affiliate: Anna Frebel

| First 5 Authors: Anirudh Chiti, Mohammad K. Mardini, Anna Frebel, Tatsuya Daniel,

| Summary:

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
$|Z|sim7$ 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 $Deltalog(N)/Delta$[Fe/H] =
1.52$pm$0.05, and shifts to $Deltalog(N)/Delta$[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 $sim5$kpc, showing that very
and extremely metal-poor stars are in every galactic component.

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