Clump-like Structures in High-Redshift Galaxies: Mass Scaling and Radial Trends from JADES

TL;DR

This study uses deep JWST/NIRCam imaging from the JADES GOODS–South field to statistically characterize clump-like substructures in roughly 3600 galaxies across $2 \lesssim z \lesssim 8$. Clumps are identified as residuals after subtracting smooth Sérsic galaxy profiles and are analyzed with watershed segmentation and Prospector-$\alpha$ SED fitting to derive stellar mass, SFR, metallicity, and age. The results show clumps are typically low-mass ($\log M_\star/M_\odot \sim 7$–$8$), actively star-forming with modest dust, and that their size and spacing grow toward lower redshift, while the clump mass function follows $\alpha = -1.50^{+0.19}_{-0.17}$, consistent with turbulent disk fragmentation. Distinct outer and inner clump scaling ($R_e \propto M_*^{0.29}$ vs $R_e \propto M_*^{0.20}$) and a central dearth of young clumps support an inside-out, migration-influenced evolution that shapes galaxy morphology during the peak epoch of star formation.

Abstract

Massive star-forming clumps are a prominent feature of high-redshift galaxies and are thought to trace gravitational fragmentation, feedback, and bulge growth in gas-rich disks. We present a statistical analysis of clump-like structures in $\sim$3600 galaxies spanning $2 \lesssim z \lesssim 8$ from deep JWST/NIRCam imaging in the JADES GOODS--South field. Clumps are identified as residual features after subtracting smooth Sérsic profiles, enabling a uniform, rest-frame optical census of sub-galactic structure. We characterize their physical properties, size--mass relations, and spatial distributions to constrain models of sub-galactic structure formation and evolution. We find that clumps in our sample are typically low-mass ($10^{\sim7-8}M_\odot$), actively star-forming, and show diverse gas-phase metallicity, dust attenuation, and stellar population properties. Their sizes and average pairwise separations increase with cosmic time (toward lower redshift), consistent with inside-out disk growth. The clump mass function follows a power law with slope $α= -1.50_{-0.17}^{+0.19}$, consistent with fragmentation in turbulent disks. We find a deficit of relatively young clumps near galaxy centers and a radial transition in the size--mass relation: outer clumps exhibit steeper, near-virial slopes ($R_{\rm e}\propto M_*^{\sim 0.3}$), while inner clumps follow flatter trends ($R_{\rm e}\propto M_*^{\sim 0.2}$), consistent with structural evolution via migration or disruption. These results provide new constraints on the formation, survival, and dynamical evolution of clumps, highlighting their role in shaping galaxy morphology during the peak of cosmic star formation.

Figures (11)

• Figure 1: Overview of the parent galaxy sample in the JADES GOODS--South field. (a) Sky distribution of galaxies color-coded by the number of identified clumps ($N_{\mathrm{clumps}}$), with clumpy galaxies highlighted. (b) Redshift distribution of the full sample. (c) Distribution of effective radii ($R_{\mathrm{e}}$) in kpc. (d) Distribution of Sérsic indices ($n$) for the full sample.
• Figure 2: Clump identification procedure for three example galaxies. From left to right: (1) three-color NIRCam image using F277W (red), F200W (green), and F115W (blue); (2) F200W image used for Sérsic profile fitting and clump detection (shown within the JADES kron aperture); (3) best-fit PSF-convolved Sérsic model generated using PySersic; (4) residual image after subtracting the model; (5) segmentation map from watershed clump detection, with clump regions shown in colors.
• Figure 3: Average number of clump-like structures per galaxy as a function of galaxy properties. Top row shows $\langle N_{\rm clumps} \rangle$ as a function of galaxy specific star formation rate (left) and stellar mass (right). Stellar mass exhibits a stronger correlation with clump number than specific star formation rate. Error bars in the top panels indicate the standard deviation of $N_{\rm clumps}$ within each bin. Bottom row shows $\langle N_{\rm clumps} \rangle$ as a function of redshift (left), Sérsic index (middle), and Gini coefficient (right). Error bars in the bottom panels represent the 16th to 84th percentile range of the binned averages. To isolate the role of stellar mass, dotted and dashed lines indicate low mass ($M_* < 10^9\,M_\odot$) and high mass ($M_* \geq 10^9\,M_\odot$) galaxies, respectively. The overall trends with redshift and morphology remain consistent across stellar mass bins.
• Figure 4: Distributions of physical properties derived from SED fitting of clump-like structures. Shown are (top row, left to right) stellar mass, star formation rate (SFR), and specific star formation rate (sSFR), and (bottom row) gas-phase metallicity, mass-weighted age, and dust attenuation parameter (dust2 from Prospector, corresponding to the diffuse dust V-band optical depth). Vertical lines mark the 16th, 50th, and 84th percentiles of each distribution. In the stellar mass panel, the red histogram shows uncorrected values, while the blue histogram accounts for the fraction of flux in the residual image relative to the original image within the segmented clump regions. Clump-like regions span a wide range of gas-phase metallicities and mass-weighted ages, indicating diverse stellar populations and evolutionary states. They are typically actively star forming, with a median $\log(\mathrm{sSFR}/\mathrm{yr}^{-1}) = -8.42$, and exhibit low to moderate dust attenuation. We caution that metallicity and age estimates are subject to significant uncertainties due to host light contamination and degeneracies in SED fitting, and should therefore be interpreted with care.
• Figure 5: Size--mass relation of clump-like structures in JADES GOODS--South, shown in three redshift bins: $z = 0$--3 (green), $z = 3$--6 (orange), and $z = 6$--9 (red). Semi-transparent points show individual clumps; large symbols mark the mean $R_{\rm e}$ in each stellar mass bin, with error bars indicating the 16th--84th percentile range. For comparison, gray squares show the host-galaxy size--mass relation measured from the same sample, gray triangles represent lensed clumps from claeyssens_star_2023 at $z \sim 1$--5, and black triangles denote NIRCam-resolved clumps from kalita_clumps_2025 at $1 < z < 2$. The host size--mass relation measured here is broadly consistent with other recent JADES results danhaive_beyond_2025 and measurements from the literature allen_galaxy_2025miller_jwst_2025yang_cosmos-web_2025. Overall, the JADES clumps occupy an intermediate regime between compact lensed star-forming knots and their host galaxies, connecting small-scale substructures to the global size--mass relation across cosmic time.