Clumpiness of galaxies revealed in the near-infrared with COSMOS-Web

Clumps in the rest-frame UV emission of galaxies have been observed for decades. Since the launch of the James Webb Space Telescope (JWST), a large population is detected in the rest-frame near-infrared (NIR), raising questions about their formation mechanism. We investigate the presence and properties of NIR over-densities (hereafter substructures) in star-forming and quiescent galaxies at 1 < z < 4 to understand their link to the evolution of their host galaxy. We identify substructures in JWST/NIRCam F277W and F444W residual images at a rest-frame wavelength of 1 um. The fraction of galaxies with substructures with M* > 10^9 Msun has been steadily decreasing with cosmic time from 40% at z = 4 to 10% at z = 1. Clumps, the main small substructures in the rest-frame NIR, are the most common type and are much fainter (2% of the flux) than similar UV clumps in the literature. Nearly all galaxies at the high-mass end of the main sequence (MS), starburst, and green valley regions have substructures. However, we do not find substructures in low-mass galaxies in the green valley and red sequence. Although massive galaxies on the MS and in the green valley have a 40% probability of hosting multiple clumps, the majority of clumpy galaxies host only a single clump. The fraction of clumpy galaxies in the rest-frame NIR is determined by the stellar mass and SFR of the host galaxies. Its evolution with redshift is due to galaxies moving towards lower SFRs at z < 2 and the build-up of low-mass galaxies in the green valley and red sequence. Based on their spatial distribution in edge-on galaxies, we infer that most of substructures are produced in-situ via disk fragmentation. Galaxy mergers may still play an important role at high stellar masses, especially at low SFR.

💡 Research Summary

This paper presents a systematic study of near‑infrared (NIR) substructures—referred to as “clumps”—in a representative sample of star‑forming and quiescent galaxies spanning 1 < z < 4, using the deep JWST/NIRCam imaging from the COSMOS‑Web survey. The authors first construct a multi‑wavelength catalog that combines JWST F115W, F150W, F277W, and F444W data with extensive ground‑based optical–near‑IR photometry (CFHT, HSC, UltraVISTA, HST/ACS). Photometric redshifts and physical parameters (stellar mass, star‑formation rate) are derived with two independent SED‑fitting codes, Le PHARE and CIGALE, showing consistent results within ~0.1 dex.

Morphologically, each galaxy is modeled with a bulge‑disk decomposition using Source Extractor++. The best‑fit structural parameters (centers, scale radii, ellipticities, position angles) are held fixed across all bands, while fluxes are fitted separately. After convolving the models with the appropriate PSF, the authors subtract them from the observed images to obtain residual maps that highlight localized over‑densities. Two detection strategies are explored: an “optimal” approach that works directly on the observed residuals, and an “intrinsic” approach that first removes the modeled smooth components. Both methods yield consistent clump catalogs.

Clumps are defined as NIR over‑densities with stellar masses M⋆ > 10⁹ M☉ and a signal‑to‑noise ≥ 5σ. The key findings are:

1. Redshift evolution of clumpy fraction – The proportion of galaxies hosting at least one clump declines sharply from ~40 % at z ≈ 4 to ~10 % at z ≈ 1. This trend mirrors the well‑known decline of specific star‑formation rate (sSFR) and the buildup of low‑mass, low‑SFR systems (green‑valley and red‑sequence galaxies) at later times.

2. Dependence on stellar mass and SFR – Almost every massive (M⋆ > 10¹⁰·⁵ M☉) galaxy on the main sequence, in the starburst region, or in the green valley exhibits at least one clump. In contrast, low‑mass galaxies (M⋆ < 10⁹·⁵ M☉) in the green valley or red sequence rarely show clumps. Thus, clump occurrence is tightly linked to the host’s position in the M⋆–SFR plane.

3. Clump luminosity contribution – NIR clumps contribute only ~2 % of the total NIR flux, considerably fainter than UV clumps reported in earlier HST studies (which can contribute 10–20 %). This suggests that NIR clumps are dominated by older stellar populations or that young stars contribute less to the NIR at high sSFR.

4. Multiplicity – About 40 % of clumpy galaxies host more than one clump, but the majority contain just two or fewer. The observed multiplicity is lower than predictions from high‑resolution hydrodynamic simulations that often generate 5–10 clumps per unstable disk, indicating possible observational blending or resolution limits.

5. Spatial distribution – In edge‑on systems, clumps are preferentially located within the disk plane rather than near the nucleus, supporting an “in‑situ” formation scenario via gravitational disk fragmentation. However, the most massive, low‑SFR galaxies show a modest excess of central clumps, hinting that minor/major mergers may contribute to clump formation in this regime.

The authors compare their results with a suite of simulations (e.g., Bournaud 2014, Perez 2013, FIRE). Simulations that include strong stellar feedback often destroy low‑mass clumps quickly, whereas those with milder feedback allow ~10⁹ M☉ clumps to survive for ≳1 Gyr and migrate inward, potentially building bulges. The observed NIR clumps, being faint and relatively few, are consistent with the latter picture: they are long‑lived, massive structures that can participate in secular bulge growth, but they do not dominate the current star‑formation budget.

In summary, this work leverages JWST’s unprecedented NIR resolution and depth to demonstrate that clumpiness is not exclusive to the UV but is a pervasive feature of high‑redshift galaxies, strongly modulated by stellar mass and star‑formation activity. The decline of the clumpy fraction with cosmic time is driven both by the overall drop in sSFR and by the emergence of low‑mass, quiescent systems. Most clumps appear to arise from internal disk instabilities, while mergers may become important for the most massive, passive galaxies. These findings provide crucial observational constraints for models of galaxy evolution, especially regarding the role of clumps in disk stability, bulge formation, and the morphological transformation of galaxies across cosmic time.