The Dawn of Disk Formation in a Milky Way-sized Galaxy Halo: Thin Stellar Disks at $z>4$

Abstract

We present results from \textsc{GigaEris}, a cosmological, $N$-body hydrodynamical "zoom-in" simulation of the formation of a Milky Way-sized galaxy halo with unprecedented resolution, encompassing of order a billion particles within the refined region. The simulation employs a modern implementation of smoothed-particle hydrodynamics, including metal-line cooling and metal and thermal diffusion. We focus on the early assembly of the galaxy, down to redshift $z=4.4$. The simulated galaxy has properties consistent with extrapolations of the main sequence of star-forming galaxies to higher redshifts and levels off to a star formation rate of $\sim$60$M_{\odot }$~yr${}^{-1}$ at $z=4.4$. A compact, thin rotating stellar disk with properties analogous to those of low-redshift systems arises already at $z\sim 8$. The galaxy rapidly develops a multi-component structure, and the disk, at least at these early stages, does not grow "upside-down" as often reported in the literature. Rather, at any given time, newly born stars contribute to sustain a thin disk. The kinematics reflect the early, ubiquitous presence of a thin disk, as a stellar disk component with $v_{\varphi }/{\sigma }_R$ larger than unity is already present at $z\sim 9$--10. Our results suggest that high-resolution spectro-photometric observations of very high-redshift galaxies should find thin rotating disks, consistent with the recent discovery of cold rotating gas disks by ALMA. Finally, we present synthetic images for the JWST NIRCam camera, showing how the early disk would be easily detectable already at those early times.

19 pages, 14 figures, 1 table, accepted to ApJ

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• Regular rotation and low turbulence in a diverse sample of z ~ 4.5 galaxies observed with ALMA
• Dynamical characterization of galaxies up to $z\sim 7$
• Born this way: thin disc, thick disc, and isotropic spheroid formation in FIRE-2 Milky-Way-mass galaxy simulations
• Formation of Galactic Disks I: Why Did the Milky Way's Disk Form Unusually Early?
• Discovery of the local counterpart of disc galaxies at z > 4: The oldest thin disc of the Milky Way using Gaia-RVS
• VINTERGATAN IV: Cosmic phases of star formation in Milky Way-like galaxies
• The imprint of clump formation at high redshift. II. The chemistry of the bulge
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• Turbulent gas-rich discs at high redshift: origin of thick stellar discs through 3D 'baryon sloshing'
• Black hole-galaxy co-evolution and the role of feedback
• The AGORA High-resolution Galaxy Simulations Comparison Project. VIII: Disk Formation and Evolution of Simulated Milky Way Mass Galaxy Progenitors at $1<z<5$