Exploring the fate of primordial discs in Milky Way-sized galaxies with the GigaEris simulation
arxiv(2024)
Abstract
Recent observations with JWST and ALMA have unveiled galaxies with regular
discs at significantly higher redshifts than previously expected. This appears
to be in contrast with constraints on the stellar populations of the Milky Way,
suggesting that the bulk of the Galactic thin disc formed after z=1, and
raises questions about the history, evolution, and survivability of primordial
discs. Here, we use GigaEris, a state-of-the-art N-body, hydrodynamical,
cosmological “zoom-in” simulation with a billion particles within the virial
radius, to delve into the formation of the early kinematically cold discs
(KCDs), defined by their ratio between the mean rotational velocity and the
radial velocity dispersion, of a Milky Way-sized galaxy at redshifts z≳
4. Our analysis reveals a primarily inward migration pattern for disc stars
formed at z ≳ 6, turning into a mix of inward and outward migration at
later times. Stars migrating outwards undergo minimal kinematic heating, and
might be identified as part of the thin disc forming at much later epochs. We
find that approximately 76 per cent of all stars formed in the KCD at z ∼
7 become part of a pseudo-bulge by z = 4.4. This proportion decreases to
below 10 per cent for KCD stars formed at z ≲ 5. The inward migration
of stars born in our KCDs at z ≳ 4 deviates from the expected
inside-out formation scenario of thin discs at lower redshifts. Our results
suggest a novel “two-phase” disc formation process, whereby the early disc
transforms primarily into the pseudo-bulge within less than a billion years,
whereas the present-day disc forms subsequently from higher-angular momentum
material accreted at later times.
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