The Kormendy Relation in the First Billion Years: Evidence from $JWST$

TL;DR

This study uses JWST data to show that spheroidal galaxies within the first billion years already follow a well-defined Kormendy relation, indicating early, dissipative formation processes.

Contribution

First observational evidence of the Kormendy relation at z≥6, revealing early galaxy structural properties and formation mechanisms.

Findings

Spheroidal galaxies at z≥6 follow a steep Kormendy relation.

Galaxies exhibit high surface brightness and compact sizes.

Results suggest rapid, dissipative assembly in high gas fraction environments.

Abstract

Galaxy scaling relations encode key information about the structural, dynamical, and mass assembly histories of galaxies, and provide constraints on galaxy formation models as well as the onset of galaxy assembly. While these relations are well characterized out to intermediate redshifts, their existence during the first billion years of cosmic history remains largely unconstrained due to observational limitations. In this work, we investigate the Kormendy relation (KR) for spheroidal systems at $z~\ge~6$ using rest-frame $B$-band structural parameters derived from publicly available deep \textit{JWST} imaging of the GOODS, CEERS, PRIMER-UDS, and PRIMER-COSMOS fields. We find that spheroidal galaxies at these epochs already occupy a well-defined locus in the mean effective surface brightness $(\langle~\mu_{\rm e}~\rangle)$ and effective radius ($\rm~R_{\rm e}$) plane, demonstrating that a KR is already in place when the universe was less a gigayear old. The best-fit relation has a slope of $\beta~=~4.25^{+0.40}_{-0.39}$ and a zero-point of $\alpha~=~15.89^{+0.17}_{-0.17}$, indicating a steeper relation and systematically higher surface brightness compared to the local relation. This steepness reflects the compact sizes and high central stellar-mass densities of these systems, consistent with rapid, dissipative assembly in environments with high gas fractions, likely driven by efficient gas inflows, and gas-rich mergers. The presence of dense bulges embedded in some of these galaxies at similar redshifts further supports a common formation pathway for both bulges and spheroids. Altogether, these findings indicate a predominantly dissipative mode of assembly for the first spheroidal systems which may evolve into the compact quiescent galaxies observed at later cosmic epochs.