Discovery of the local counterpart of disc galaxies at z > 4: The oldest thin disc of the Milky Way using Gaia-RVS

Abstract

JWST has recently detected numerous disc galaxies at high-redshifts and there have been observations of cold disc galaxies at z > 4 with ALMA. In the Milky Way, recent studies find metal-poor stars in cold disc orbits, suggesting an ancient disc. We investigated a sample of 565,606 stars from the hybrid-CNN analysis of the Gaia-DR3 RVS stars. The sample contains 8,500 stars with [Fe/H]<-1. For a subset of ~200,000 MSTO and subgiant stars we computed distances and ages using the StarHorse code with a mean precision of 1% and 12%, respectively. First, we confirm the existence of metal-poor stars in thin disc orbits - over 50% are older than 13 Gyr. Second, we report the discovery of the oldest thin disc of the Milky Way(MW), which extends across a wide range of metallicities, from metal-poor to super-solar stars. The metal-poor stars in disc orbits manifest as a readily visible tail of the metallicity distribution. The high-[α/Fe] thick disc exhibits a vertical velocity dispersion of 35 km/s, while the thin disc shows 10 to 15 km/s lower at similar ages. Our old thin disc ${\sigma }_{V_z}$ appears similar to those estimated for the high-z disc galaxies. Third, we extend the [Y/Mg] chemical clock to the oldest ages and estimate a slope of -0.038 dex/Gyr. Finally, we confirm our discovery by showing that the splash includes high- and low-[α/Fe] populations that are both old and extends to super-solar [Fe/H]. We find about 6 to 10% of the old thin disc was heated to thick disc orbits with the youngest splashed stars being 9 to 10 Gyrs. We conclude the MW thin disc forms <1 billion years from Big Bang, building up inside-out, preceding earlier estimates by about 4-5 billion years. Considering a massive merger event such as the GSE, a Splash is expected - we find a portion of the old thin disc is heated to thick disc velocities and the Splash extends to super-solar [Fe/H] regimes.

Accepted for publication in A&A. The abstract has been substantially modified to comply with arXiv's word limit, please refer to the pdf

References in corpus (48)

• Array Programming with NumPy
• The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package
• Gaia Data Release 3: Summary of the content and survey properties
• galpy: A Python Library for Galactic Dynamics
• The mass distribution and gravitational potential of the Milky Way
• Gaia Early Data Release 3: Parallax bias versus magnitude, colour, and position
• Gaia Early Data Release 3: Photometric content and validation
• The GALAH+ Survey: Third Data Release
• Gaia Data Release 3: Analysis of RVS spectra using the General Stellar Parametriser from spectroscopy
• A time-resolved picture of our Milky Way's early formation history
• Gaia Data Release 3: Astrophysical parameters inference system (Apsis) I -- methods and content overview
• Gaia Data Release 3: Processing and validation of BP/RP low-resolution spectral data
• From dawn till disk: Milky Way's turbulent youth revealed by the APOGEE+Gaia data
• Age dissection of the Milky Way discs: red giants in the Kepler field
• A dynamically cold disk galaxy in the early Universe
• Reverse engineering the Milky Way
• Reconstructing the Last Major Merger of the Milky Way with the H3 Survey
• Dynamical properties of z $\sim 4.5$ dusty star-forming galaxies and their connection with local early type galaxies
• CEERS Key Paper III: The Diversity of Galaxy Structure and Morphology at z=3-9 with JWST
• Panic! At the Disks: First Rest-frame Optical Observations of Galaxy Structure at $z>3$ with JWST in the SMACS 0723 Field
• The chemical characterisation of halo substructure in the Milky Way based on APOGEE
• A Cold, Massive, Rotating Disk Galaxy 1.5 Billion Years after the Big Bang
• The TESS-HERMES survey Data Release 1: high-resolution spectroscopy of the TESS southern continuous viewing zone
• Chronologically dating the early assembly of the Milky Way
• High-precision abundances of elements in solar-type stars. Evidence of two distinct sequences in abundance-age relations
• StarHorse results for spectroscopic surveys + Gaia DR3: Chrono-chemical populations in the solar vicinity, the genuine thick disk, and young-alpha rich stars
• PLATO as it is: a legacy mission for Galactic archaeology
• What Causes The Formation of Disks and End of Bursty Star Formation?
• The Pristine survey X: a large population of low-metallicity stars permeates the Galactic disk
• Reconstructing the Disrupted Dwarf Galaxy $Gaia$-Sausage/Enceladus Using its Stars and Globular Clusters
• Chemical Abundances of Main-Sequence, Turnoff, Subgiant and red giant Stars from APOGEE spectra II: Atomic Diffusion in M67 Stars
• Spiral morphology in an intensely star-forming disk galaxy more than 12 billion years ago
• Morpheus Reveals Distant Disk Galaxy Morphologies with JWST: The First AI/ML Analysis of JWST Images
• The Gaia DR3 view of dynamical substructure in the stellar halo near the Sun
• Regular rotation and low turbulence in a diverse sample of z ~ 4.5 galaxies observed with ALMA
• Co-formation of the thin and thick discs revealed by APOGEE-DR16 and Gaia-DR2
• The Atari Disk, a Metal-Poor Stellar Population in the Disk System of the Milky Way
• Dynamically cold disks in the early Universe: myth or reality?
• Disk settling and dynamical heating: histories of Milky Way-mass stellar disks across cosmic time in the FIRE simulations
• Cold gas disks in main-sequence galaxies at cosmic noon: Low turbulence, flat rotation curves, and disk-halo degeneracy
• The Pristine survey XIII: Uncovering the very metal-poor tail of the thin disc
• The Dawn of Disk Formation in a Milky Way-sized Galaxy Halo: Thin Stellar Disks at $z>4$
• Ancient Very Metal-Poor Stars Associated With the Galactic Disk in the H3 Survey
• Understanding the early stages of galaxy formation using very metal-poor stars from the Hamburg/ESO survey
• On the metal-poor edge of the Milky Way "thin disc"
• On the likelihoods of finding very metal-poor (and old) stars in the Milky Way's disc, bulge, and halo
• A Very Metal-poor RR Lyrae Star with a Disk Orbit Found in the Solar Neighborhood
• Exploring the dependence of chemical traits on metallicity: chemical trends for red giant stars with asteroseismic ages

Cited by in corpus (11)

• Chronology of our Galaxy from Gaia Colour-Magnitude Diagram-fitting (ChronoGal). I. The formation and evolution of the thin disk from the Gaia Catalogue of Nearby Stars
• Transferring spectroscopic stellar labels to 217 million Gaia DR3 XP stars with SHBoost
• The Pristine survey: XXV. The very metal-poor Galaxy: Chemodynamics through the follow-up of the Pristine-Gaia synthetic catalogue
• Chronology of our Galaxy from Gaia colour-magnitude diagram fitting (ChronoGal) II. Unveiling the formation and evolution of the kinematically selected Thick and Thin Discs
• Pushing the boundaries of asteroseismic individual frequency modelling: unveiling two evolved very-low metallicity red giants
• The oldest Milky Way stars: New constraints on the age of the Universe and the Hubble constant
• The Spheroidal Bulge of the Milky Way: Chemodynamically Distinct from the Inner-Thick Disc and Bar
• The metal-poor tail of the APOGEE survey I. Uncovering [Fe/H] < -2.5 stars from the inner Galaxy to the Magellanic Clouds
• An ancient system hidden in the Galactic plane?
• Probing the Milky Way Halo with RR Lyrae Stars from Gaia Data Release 3
• Constraints on the gravitational potential from DESI DR2 BAO and its implications for the local void scenario