Kavli Affiliate: Roberto Maiolino
| Summary:
We measure the fraction of clumpy galaxies ($f_rm clumpy$) over the redshift range $z=1-9$ and stellar masses $log_10(M_star/rm M_odot) geq 8$ 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 $f_rm clumpy$, alongside a probability-weighted completeness-corrected statistic for comparison. For the full sample, the blue and optical $f_rm clumpy$ reach $approx55$ per cent at $zapprox1-2$ and decline to $lesssim5$ per cent by $zapprox8-9$, while the UV fraction remains higher at $approx12$ per cent. At $zapprox1-2$, $f_rm clumpy$ increases strongly with stellar mass, from $approx35$ per cent in the lowest-mass bin to $approx85$ per cent at $log_10(M_star/rm M_odot)geq10$. 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 ($log_10(M_star/rm M_odot)<8.5$), high-redshift clumpy galaxies, while the observed fractions in more massive systems ($log_10(M_star/rm M_odot)>9$) 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.
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