The Pristine Inner Galaxy Survey (PIGS) VIII: Characterising the orbital properties of the ancient, very metal-poor inner Milky Way
arxiv(2023)
Abstract
The oldest stars in the Milky Way (born in the first few billion years) are
expected to have a high density in the inner few kpc, spatially overlapping
with the Galactic bulge. We use spectroscopic data from the Pristine Inner
Galaxy Survey (PIGS) to study the dynamical properties of ancient, metal-poor
inner Galaxy stars. We compute distances using StarHorse, and orbital
properties in a barred Galactic potential. With this paper, we release the
spectroscopic AAT/PIGS catalogue (13 235 stars). We find that most PIGS stars
have orbits typical for a pressure-supported population. The fraction of stars
confined to the inner Galaxy decreases with decreasing metallicity, but many
very metal-poor stars (VMP, [Fe/H] < -2.0) stay confined ( 60
kpc). The azimuthal velocity v_ϕ also decreases between [Fe/H] = -1.0 and
-2.0, but is constant for VMP stars (at 40 km/s). The carbon-enhanced
metal-poor (CEMP) stars in PIGS appear to have similar orbital properties
compared to normal VMP stars. Our results suggest a possible transition between
two spheroidal components - a more metal-rich, more concentrated, faster
rotating component, and a more metal-poor, more extended and
slower/non-rotating component. We propose that the former may be connected to
pre-disc in-situ stars (or those born in large building blocks), whereas the
latter may be dominated by contributions from smaller galaxies. This is an
exciting era where large metal-poor samples, such as in this work (as well as
upcoming surveys, e.g., 4MOST), shed light on the earliest evolution of our
Galaxy.
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