NIHAO XIII: Clumpy discs or clumpy light in high redshift galaxies?
  Many massive star forming disc galaxies in the redshift range 3 to 0.5 are
observed to have a clumpy morphology showing giant clumps of size $\sim$1 kpc
and masses of about $10^7M_{\odot}$ to $10^{10} M_{\odot}$. The nature and fate
of these giant clumps is still under debate. In this work we use 19
high-resolution simulations of disc galaxies from the NIHAO sample to study the
formation and the evolution of clumps in the discs of high redshift galaxies.
We use mock HST - CANDELS observations created with the radiative transfer code
GRASIL-3D to carry out, for the first time, a quantitative comparison of the
observed fraction of clumpy galaxies and its evolution with redshift with
simulations. We find a good agreement between the observed clumpy fraction and
the one of the NIHAO galaxies. We find that dust attenuation can suppress
intrinsically bright clumps and enhance less luminous ones. In our galaxy
sample we only find clumps in light (u-band) from young stars but not in
stellar mass surface density maps. This means that the NIHAO sample does not
show clumpy stellar discs but rather a clumpy light distribution originating
from clumpy star formation events. The clumps found in the NIHAO sample match
observed age/color gradients as a function of distance from the galaxy center
but they show no sign of inward migration. Clumps in our simulations disperse
on timescales of a about a hundred Myr and their contribution to bulge growth
is negligible.
