  While the formation of stellar clumps in distant galaxies is usually
attributed to gravitational violent disk instabilities, we show here that major
mergers also represent a competitive mechanism to form bright clumps. Using
~0.1'' resolution ACS F814W images in the entire COSMOS field, we measure the
fraction of clumpy emission in 109 main sequence (MS) and 79 Herschel-detected
starbursts (off-MS) galaxies at 0.5 < z < 0.9, representative of normal versus
merger induced star-forming activity, respectively. We additionally identify
merger samples from visual inspection and from Gini-M20 morphological
parameters. Regardless of the merger criteria adopted, the clumpiness
distribution of merging systems is different from that of normal isolated disks
at > 99.5 % confidence level, with the former reaching higher clumpiness
values, up to 20 % of the total galaxy emission. We confirm the merger induced
clumpiness enhancement with novel hydrodynamical simulations of colliding
galaxies with gas fractions typical of z ~ 0.7. Multi-wavelength images of
three starbursts in the CANDELS field support the young nature of clumps, which
are likely merger products rather than older pre-existing structures. Finally,
for a subset of 19 starbursts with existing near-IR rest frame spectroscopy, we
find that the clumpiness is mildly anti-correlated with the merger phase,
decreasing towards final coalescence. Our result can explain recent ALMA
detections of clumps in hyperluminous high-z starbursts, while normal objects
are smooth. This work raises a question on the role of mergers on the origin of
clumps in high redshift galaxies in general.
