Formation and Morphology of the First Galaxies in the Cosmic Morning

Changbom Park, Jaehyun Lee, Juhan Kim, Donghui Jeong, Christophe Pichon, Brad K. Gibson, Owain N. Snaith, Jihye Shin, Yonghwi Kim, Yohan Dubois, C. Gareth Few

Published 2022-02-24Version 1

We investigate the formation and morphological evolution of the first galaxies in the cosmic morning ($10>z>4$) using the Horizon Run 5 (HR5) cosmological hydrodynamical simulation. For galaxies above the stellar mass $M_{\star} = 2\times10^9\,M_{\odot}$, we classify them into disk, spheroid, and irregular types according to their asymmetry and stellar mass morphology. We find that 2/3 of galaxies have a S\'{e}rsic index less than 1.5, reflecting the dominance of disk-type morphology in the cosmic morning. The rest are evenly distributed as irregulars or spheroids. We also find that these fractions are roughly independent of redshift and stellar mass up to $\sim10^{10}\,M_{\odot}$, while irregular or spheroidal morphology appears incidental and transient. Almost all the first galaxies with $M_{\star}>2\times10^9\,M_{\odot}$ at redshift 6 form at initial peaks of the matter density field. Large-scale structures in the universe emerge and grow like cosmic rhizomes as the underlying matter density fluctuations grow and form associations of galaxies in rare overdense regions. The growth of the density field further stretches the realm of the galactic world into relatively lower-density regions along evolving filaments. The cosmic web of galaxies forms at lower redshifts when most rhizomes globally percolate. The primordial angular momentum produced by the induced tidal torques on protogalactic regions is correlated with the internal kinematics of galaxies and tightly aligned with the angular momentum of the total galaxy mass. However, the primordial angular momentum only very weakly correlates with the instantaneous morphology and orientation of the stellar component below $z=6$. The large-scale tidal field imprinted in the initial conditions seems responsible for the dominance of disk morphology, and for the tendency of galaxies to re-acquire a disk post-distortion.