Kavli Affiliate: Andrey Kravtsov
| First 5 Authors: Elliot Y. Davies, Vasily Belokurov, Andrey Kravtsov, Stephanie Monty, GyuChul Myeong
| Summary:
The stellar halo of the Milky Way comprises an abundance of chemical
signatures from accretion events and textit{in-situ} evolution, that form an
interweaving tapestry in kinematic space. To untangle this, we consider the
mixtures of chemical information, in a given region of integral of motion
space, as a variant of the blind source separation problem and utilise
non-negative matrix factorisation (NMF). Specifically, we examine the variation
in [Fe/H], [Mg/Fe], and [Al/Fe] distributions of APOGEE DR17 stars across the
$(E,L_z)$ plane of the halo. When 2 components are prescribed, the NMF
algorithm splits stellar halo into low- and high-energy components in the
$(E,L_z)$ plane which approximately correspond to the accreted and
textit{in-situ} halo respectively. We use these two components to define a new
boundary between the textit{in-situ} and the accreted stellar halo. Moreover,
we calculate the components fractional contribution to the stellar halo as a
function of energy, galactocentric spherical radius, height, and galactocentric
cylindrical radius. Using a stellar halo defined by kinematic cuts, we find
that the halo transitions from textit{in-situ} dominated to accretion
dominated at $E approx -1.67 times 10^5$ (km/s)$^2$ (using the potential in
McMillan 2017), and at $(r,z,R) approx (8.7, 3.0, 8.1)$ kpc. The low-energy
component is found to span a range of [Al/Fe] that falls beyond the typically
accepted textit{in-situ} floor of [Al/Fe] $=0$. Upon prescribing more
components to the NMF model, we find hints of the existence of overlapping
chemical evolution sequences that other techniques struggle to find. We also
examine features within these components that resemble known substructures in
the halo, such as textit{Eos} and textit{Aurora}. This work provides insight
into their origin and the part they play in the Milky Way’s formation.
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