The clumpy fraction of galaxies at $z=1-9$ with JADES: evidence for a mass-dependent transition in clump origin

We measure the fraction of clumpy galaxies ($f_{\mathrm{c}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{p}\mathrm{y}}$) over the redshift range $z=1-9$ and stellar masses ${\log }_{10}(M_{\star }\mathrm{/}{\mathrm{M}}_{\odot })\ge 8$ using JWST/NIRCam imaging from JADES. We define clumps as compact off-centre surface-brightness peaks with characteristic sizes of a few hundred parsecs, and develop a dendrogram-based clump-finding method with astrodendro to identify them directly in rest-frame UV, blue, and optical images. Clump-detection completeness is quantified through injection-recovery simulations, which we use to define ($M_{\star }$, $z$)-dependent 50 per cent completeness-limited $f_{\mathrm{c}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{p}\mathrm{y}}$, alongside a probability-weighted completeness-corrected statistic for comparison. For the full sample, the blue and optical $f_{\mathrm{c}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{p}\mathrm{y}}$ reach $\approx 55$ per cent at $z\approx 1-2$ and decline to $\lesssim 5$ per cent by $z\approx 8-9$, while the UV fraction remains higher at $\approx 12$ per cent. At $z\approx 1-2$, $f_{\mathrm{c}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{p}\mathrm{y}}$ increases strongly with stellar mass, from $\approx 35$ per cent in the lowest-mass bin to $\approx 85$ per cent at ${\log }_{10}(M_{\star }\mathrm{/}{\mathrm{M}}_{\odot })\ge 10$. Most clumpy galaxies host only one or two robustly detected off-centre clumps, while higher-multiplicity systems occur preferentially in massive, strongly star-forming galaxies at low to intermediate redshift. Comparison with major merger rates suggests that mergers could account for a substantial fraction of low-mass (${\log }_{10}(M_{\star }\mathrm{/}{\mathrm{M}}_{\odot })<8.5$), high-redshift clumpy galaxies, while the observed fractions in more massive systems (${\log }_{10}(M_{\star }\mathrm{/}{\mathrm{M}}_{\odot })>9$) require an additional in situ clump-formation channel. Our results are consistent with a transition from merger-dominated clumpy morphologies at early times and low stellar masses to an increasing contribution from in situ clump-formation channels, such as disc instabilities, in more massive systems.