Origin of Giant Stellar Clumps in High-Redshift Galaxies
  We examine the nature of kpc-scale clumps seen in high-redshift galaxies
using a suite of cosmological simulations of galaxy formation. We identify
rest-frame UV clumps in mock HST images smoothed to 500 pc resolution, and
compare them with the intrinsic 3D clumps of young stars identified in the
simulations with 100 pc resolution. According to this comparison, we expect
that the stellar masses of the observed clumps are overestimated by as much as
an order of magnitude, and that the sizes of these clumps are also
overestimated by factor of several, due to a combination of spatial resolution
and projection. The masses of young stars contributing most of the UV emission
can also be overestimated by factor of a few. We find that most clumps of young
stars present in a simulation at one time dissolve on a timescale shorter than
$\sim$150 Myr. Some clumps with dense cores can last longer but eventually
disperse. Most of the clumps are not bound structures, with virial parameter
$\alpha_{\rm vir}$ > 1. We find similar results for clumps identified in mock
maps of H$\alpha$ emission measure. We examine the predictions for effective
clump sizes from the linear theory of gravitational perturbations and conclude
that they are inconsistent with being formed by global disc instabilities.
Instead, the observed clumps represent random projections of multiple smaller
star-forming regions.
