SCALES. II. The Internal Composition of Giant Star-Forming Clumps at $z = 1 - 4$

Abstract

Star-forming structures of scale $\sim0.5-1\,\mathrm{kpc}$, dubbed 'giant' star-forming clumps, are routinely detected in unlensed surveys of $z > 1$ galaxies. Two scenarios are usually invoked to explain their existence -- genuine physical structures or resolution-driven artifacts -- yet distinguishing between them requires characterizing their internal structure. In this work, part of the Star-Forming Clumps As Layered Emergent Structures (SCALES) series, we hence ask: What is inside a giant clump? We analyze 23 lensed galaxies at $z=1-4$ with JWST/NIRCam using difference-of-Gaussian decomposition to model (lensed) clumps at $\sim 100-300\,\mathrm{pc}$ scales, and the surrounding extended component (EC) at giant clump scales. Associating clumps to ECs, we effectively identify 44 giant clumps. Characterizing giant clump stellar mass composition using the most massive member clump and the remaining constituent clumps, we find it consistent with random sampling of the host galaxy's clump population, and hence the clump stellar mass function. This conclusion agrees qualitatively with both prevailing scenarios -- (I) dominant long-lived giant clumps, although requiring constant renewal of their constituents, and (II) giant clumps as mere blended clump associations. However, most models make no strong claims about their internal mass distribution beyond order-of-magnitude statements, and our observations thus provide an empirical constraint against which such models can be re-evaluated. Finally, using the total mass budget of clumps and ECs, we find that the clump stellar mass function cannot be described by a single power law of slope $\approx 2$, but requires a break to a shallower slope at $\sim 10^{7.4}\,\mathrm{M_{\odot}}$.