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Rapid bulge assembly in young galaxy disks at Cosmic Dawn

Anshuman Borgohain, Kanak Saha

TL;DR

This study leverages JWST imaging to quantify bulge-disk assembly in galaxies at $z\geq6$ via 2D light-profile fitting. Through SED modelling with CIGALE and GALFIT-based bulge+disk decompositions, the authors find a subset of compact, bulge-bearing systems with high $B/T$ ($\approx0.47$) and elevated central densities, indicating rapid inner mass assembly and potential early quenching. The results imply inside-out growth and suggest these early bulge-disk galaxies are progenitors of later massive star-forming and quiescent populations, shaped by dissipative processes, disk growth, compaction, and SMBH activity. Overall, the work establishes that bulge components begin assembling within the first Gyr of cosmic history, challenging simple hierarchical growth pictures and highlighting the role of rapid, central mass buildup for galaxy evolution.

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.

Rapid bulge assembly in young galaxy disks at Cosmic Dawn

TL;DR

This study leverages JWST imaging to quantify bulge-disk assembly in galaxies at via 2D light-profile fitting. Through SED modelling with CIGALE and GALFIT-based bulge+disk decompositions, the authors find a subset of compact, bulge-bearing systems with high () and elevated central densities, indicating rapid inner mass assembly and potential early quenching. The results imply inside-out growth and suggest these early bulge-disk galaxies are progenitors of later massive star-forming and quiescent populations, shaped by dissipative processes, disk growth, compaction, and SMBH activity. Overall, the work establishes that bulge components begin assembling within the first Gyr of cosmic history, challenging simple hierarchical growth pictures and highlighting the role of rapid, central mass buildup for galaxy evolution.

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 M kpc ) - values close to those of nearby quiescent galaxies. Combined with their intense central star formation rate surface densities (~ 1.26*10 M yr kpc ), 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.

Paper Structure

This paper contains 13 sections, 3 equations, 33 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Data and Sample selection
  3. Spectral energy distribution modeling
  4. 2D modelling
  5. Single component fit
  6. Double-component fit
  7. Results
  8. Bulge size vs galaxy stellar mass
  9. Evolution of Bulge size and bulge-to-total light (B/T)
  10. Galaxy main sequence at $z>$ 6
  11. Summary and Conclusions
  12. All galaxies modelled with a bulge and exponential disk components
  13. Results from GALFIT runs on mock objects

Figures (33)

  • Figure 1: Example of morphological decomposition carried out for galaxies in the sample. Top left: The grayscale images of the galaxy in the appropriate JWST filter probing rest-frame U and B band emission. The black dotted-ellipses mark the S/N$\sim$3 extent of the galaxy in the respective filters. Top right: The modelled rest-frame SED of the galaxy. Bottom left: The observed and modelled surface brightness profiles using a single Single model. The solid curves represents the modelled profiles (PSF convolved model) fitted to the observed datapoints. The dotted curves represents the intrinsic models of the galaxy in the respective filter. The dashed curve represents the PSF profile in the appropriate JWST filter. The upside down arrows represent the extent that enclose 80% of the PSF flux. These are represented below by vertical lines in the panel for the residual profiles, $\Delta\mu$. Bottom right: Same as bottom left panel, but using an inner Sersic + Exponential Disk model for the galaxy. The errorbars represent 1-$\sigma$ errors.
  • Figure 2: False-colour R (F444W), G (filter that samples rest-frame B band emission), B (F115W) images of a few best cases of galaxies modelled with an inner Sersic and underlying disk component. The vertical lines in the images span an angular size of 1".
  • Figure 3: Distribution of half-light sizes and Sersic indices (from single Sersic modeling) vs the stellar masses of the galaxies.
  • Figure 4: Global galaxy properties of $z>$ 6 galaxies (containing a bulge and a disk) from single-Sersic modeling and comparison to low-$z$ counterparts in the CANDELS fields dimauro-etal2018.
  • Figure 5: Bulge sizes for the galaxies modelled with two components vs their total stellar mass.
  • ...and 28 more figures