Metallicity distributions of halo stars: do they trace the Galactic accretion history?
arxiv(2024)
摘要
The standard cosmological scenario predicts a hierarchical formation for
galaxies. Many substructures were found in the Galactic halo, identified as
clumps in kinematic spaces, like the energy-angular momentum one (E-Lz), under
the hypothesis of the conservation of these quantities. If these clumps also
feature different chemical properties, e.g. metallicity distribution functions
(MDF), they are often associated to independent merger debris. The aim of this
study is to explore to what extent we can couple kinematics and metallicities
of stars in the Galactic halo to reconstruct the accretion history of the Milky
Way. In particular, we want to understand whether different clumps in the E-Lz
space with different MDF should be associated to distinct merger debris. We
analysed dissipationless, self-consistent high-resolution N-body simulations of
a MW-type galaxy accreting a satellite with mass ratio 1:10, with different
orbital parameters and metallicity gradients (assigned a posteriori). We
confirm that accreted stars from a 1:10 satellite redistribute in a wide range
of E and Lz, due to the dynamical friction, thus not being associated to a
single clump. Because satellite stars with different metallicities can be
deposited in different regions of the E-Lz space (on average the more
metal-rich ones end up more gravitationally bound to the MW), this implies that
a single 1:10 accretion can manifest with different MDFs, in different regions
of the E-Lz space. Groups of stars with different E, Lz and metallicities may
be interpreted as originating from different satellites, but our analysis shows
that these interpretations are not physically motivated. In fact, the coupling
of kinematics with MDFs to reconstruct the accretion history of the MW can bias
the reconstructed merger tree towards increasing the number of past accretions
and decreasing the masses of the progenitor galaxies.
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