Rapid Bulge Assembly in Young Galaxy Disks at Cosmic Dawn
- 1Inter-University Centre for Astronomy and Astrophysics
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 remain challenging due to limited spatial resolution and sensitivity. In this work, we present a clear and robust morphological analysis of a sample of 187 galaxies at z ≥ 6, demonstrating that distinct bulge and disk components were already beginning to emerge during this early epoch. Using multicomponent light-profile fitting, we identify a subset of 20 galaxies whose surface-brightness distributions can be modeled with an inner spheroidal (Sérsic) component and an underlying exponential disk component. These systems exhibit high bulge-to-total (B/T) light ratios (∼0.47) and central stellar-mass surface densities, ∼7.82 × 10 ⁸ M ⊙ kpc ⁻² —close to those of nearby quiescent galaxies. Combined with their intense central star formation rate surface densities, ∼1.21 × 10 ¹ M ⊙ 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 coevolution, driven by both internal dynamics and external interactions.
Related
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- On The Scaling Relations Between Black Holes And The Galaxy/Bulge Mass And Their Connections With The Black Hole Occupation Fraction
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- The assembly of bulge-dominated galaxies: two evolutionary channels traced through morphology, kinematics, and environment with a hybrid classification pipeline
BibTeX
@article{borgohain2026rapid,
title = {{Rapid Bulge Assembly in Young Galaxy Disks at Cosmic Dawn}},
author = {Anshuman Borgohain and Kanak Saha},
journal = {The Astrophysical Journal Supplement Series},
year = {2026},
month = oct,
doi = {10.3847/1538-4365/aea15d},
url = {https://iopscience.iop.org/article/10.3847/1538-4365/aea15d},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Rapid Bulge Assembly in Young Galaxy Disks at Cosmic Dawn
AU - Anshuman Borgohain
AU - Kanak Saha
PY - 2026
DA - 2026/10/16
JO - The Astrophysical Journal Supplement Series
DO - 10.3847/1538-4365/aea15d
UR - https://iopscience.iop.org/article/10.3847/1538-4365/aea15d
AB - 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 remain challenging due to limited spatial resolution and sensitivity. In this work, we present a clear and robust morphological analysis of a sample of 187 galaxies at z ≥ 6, demonstrating that distinct bulge and disk components were already beginning to emerge during this early epoch. Using multicomponent light-profile fitting, we identify a subset of 20 galaxies whose surface-brightness distributions can be modeled with an inner spheroidal (Sérsic) component and an underlying exponential disk component. These systems exhibit high bulge-to-total (B/T) light ratios (∼0.47) and central stellar-mass surface densities, ∼7.82 × 10 ⁸ M ⊙ kpc ⁻² —close to those of nearby quiescent galaxies. Combined with their intense central star formation rate surface densities, ∼1.21 × 10 ¹ M ⊙ 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 coevolution, driven by both internal dynamics and external interactions.
SN - 0067-0049
SN - 1538-4365
ER -