Rapid bulge assembly in young galaxy disks at Cosmic Dawn

Abstract

Recent observations with the James Webb Space Telescope (JWST) have begun to reveal a surprising morphological diversity in galaxies within the first billion years after the Big Bang, including indications of structural maturity previously thought to arise much later. These findings raise fundamental questions about when and how well-known structural components of galaxy morphology, such as bulges and disks, first emerged. However, directly identifying and resolving such structures at z $>$ 6 remains challenging due to limited spatial resolution and sensitivity. In this work, we present a clear and robust morphological analysis of a sample of 190 galaxies at z $>=$ 6, demonstrating that distinct bulge and disk components were already beginning to emerge during this early epoch. Using multi-component light profile fitting, we model the radial brightness distributions of a subset (20) of galaxies with an inner spheroidal (Sersic) component and an underlying exponential disk. These systems exhibit high bulge-to-total (B/T) light ratios (~ 0.47) and central stellar mass surface densities (~ 2.82*10$^{8}$ M$_{sun}$ kpc$^{-2}$ ) - values close to those of nearby quiescent galaxies. Combined with their intense central star formation rate surface densities (~ 1.26*10$^{1}$ M$_{sun}$ yr$^{-1}$ kpc$^{-2}$ ), our results indicate a rapid building of inner stellar mass and bulge assembly within these young systems. We propose that these early bulge-disk galaxies represent progenitors of massive star-forming and quiescent systems observed at lower redshifts. Their subsequent evolution may proceed through physical processes such as disk growth, compaction, quenching, or bulge-disk co-evolution, driven by both internal dynamics and external interactions.