Diverse Structural Evolution at z > 1 in Cosmologically Simulated
  Galaxies
  From mock Hubble Space Telescope images, we quantify non-parametric
statistics of galaxy morphology, thereby predicting the emergence of
relationships among stellar mass, star formation, and observed rest-frame
optical structure at 1 < z < 3. We measure automated diagnostics of galaxy
morphology in cosmological simulations of the formation of 22 central galaxies
with 9.3 < log10 M_*/M_sun < 10.7. These high-spatial-resolution zoom-in
calculations enable accurate modeling of the rest-frame UV and optical
morphology. Even with small numbers of galaxies, we find that structural
evolution is neither universal nor monotonic: galaxy interactions can trigger
either bulge or disc formation, and optically bulge-dominated galaxies at this
mass may not remain so forever. Simulated galaxies with M_* > 10^10 M_sun
contain relatively more disc-dominated light profiles than those with lower
mass, reflecting significant disc brightening in some haloes at 1 < z < 2. By
this epoch, simulated galaxies with specific star formation rates below 10^-9.7
yr^-1 are more likely than normal star-formers to have a broader mix of
structural types, especially at M_* > 10^10 M_sun. We analyze a cosmological
major merger at z ~ 1.5 and find that the newly proposed MID morphology
diagnostics trace later merger stages while G-M20 trace earlier ones. MID is
sensitive also to clumpy star-forming discs. The observability time of typical
MID-enhanced events in our simulation sample is less than 100 Myr. A larger
sample of cosmological assembly histories may be required to calibrate such
diagnostics in the face of their sensitivity to viewing angle, segmentation
algorithm, and various phenomena such as clumpy star formation and minor
mergers.
