The Tumultuous Formation of the Hubble Sequence at z > 1 Examined with
  HST/WFC3 Observations of the Hubble Ultra Deep Field
  We examine in this paper a stellar mass selected sample of galaxies at 1 < z
< 3 within the Hubble Ultra Deep Field, utilising WFC3 imaging to study the
rest-frame optical morphological distribution of galaxies at this epoch. We
measure how apparent morphologies (disk, elliptical, peculiar) correlate with
physical properties, such as quantitative structure and spectral-types. One
primary result is that apparent morphology does not correlate strongly with
stellar populations, nor with galaxy structure at this epoch, suggesting a
chaotic formation history for Hubble types at z > 1. By using a locally defined
definition of disk and elliptical galaxies based on structure and
spectral-type, we find no true ellipticals at z > 2, and a fraction of
3.2+/-2.3% at 1.5 < z < 2. Local counterparts of disk galaxies are at a similar
level of 7-10%, much lower than the 75% fraction at lower redshifts. We further
compare WFC3 images with the rest-frame UV view of galaxies from ACS imaging,
showing that galaxies imaged with ACS that appear peculiar often contain an
`elliptical' like morphology in WFC3. We show through several simulations that
this larger fraction of elliptical-like galaxies is partially due to the
courser PSF of WFC3, and that the `elliptical' class very likely includes
early-type disks. We also measure the merger history for our sample using CAS
parameters, finding a redshift evolution increasing with redshift, and a peak
merger fraction of ~30% at z~2 for the most massive galaxies with M_*> 10^{10}
M_sol, consistent with previous results from ACS and NICMOS. We compare our
results to semi-analytical model results and find a relatively good agreement
between our morphological break-down and the predictions. Finally, we argue
that the peculiars, ellipticals and peculiar ellipticals have similar
properties, suggesting similar formation modes, likely driven by major mergers.
