  Giant clumps are a characteristic feature of observed high-redshift disk
galaxies. We propose that these kpc-sized clumps have a complex substructure
and are the result of many smaller clumps self-organizing themselves into clump
clusters (CC). This bottom-up scenario is in contrast to the common top-down
view that these giant clumps form first and then sub fragment. Using a high
resolution hydrodynamical simulation of an isolated, fragmented massive gas
disk and mimicking the observations from Genzel et al. (2011) at $z \sim 2$, we
find remarkable agreement in many details. The CCs appear as single entities of
sizes $R_{\mathrm{HWHM}} \simeq 0.9-1.4$ kpc and masses $\sim 1.5-3 \times 10^9
\ \mathrm{M_{\odot}}$ representative of high-z observations. They are organized
in a ring around the center of the galaxy. The origin of the observed clumps'
high intrinsic velocity dispersion $\sigma_{\mathrm{intrinsic}} \simeq 50 - 100
\ \mathrm{km \ s^{-1}}$ is fully explained by the internal irregular motions of
their substructure in our simulation. No additional energy input, e.g. via
stellar feedback, is necessary. Furthermore, in agreement with observations, we
find a small velocity gradient $V_{\mathrm{grad}} \simeq 8 - 27 \ \mathrm{km \
s^{-1} \ kpc^{-1}}$ along the CCs in the beam smeared velocity residual maps
which corresponds to net prograde and retrograde rotation with respect to the
rotation of the galactic disk. The CC scenario could have strong implications
for the internal evolution, lifetimes and the migration timescales of the
observed giant clumps, bulge growth and AGN activity, stellar feedback and the
chemical enrichment history of galactic disks.
