  We have used deep, HST, near-IR imaging to study the morphological properties
of the most massive galaxies at high z, modelling the WFC3/IR H-band images of
the ~200 galaxies in the CANDELS-UDS field with 1 < z_phot < 3, and stellar
masses M_star > 10^11 M_sun. We have used both single-Sersic and bulge+disk
models, have investigated the errors/biases introduced by uncertainties in the
background and the PSF, and have obtained formally-acceptable model fits to
>90% of the galaxies. Our results indicate that these massive galaxies at 1 < z
< 3 lie both on and below the local size-mass relation, with a median R_e~2.6
kpc, a factor of ~2.3 smaller than comparably-massive local galaxies. Moreover,
we find that bulge-dominated objects in particular show evidence for a growing
bimodality in the size-mass relation with increasing z, and by z > 2 the
compact bulges display effective radii a factor ~4 smaller than local
ellipticals of comparable mass. These trends appear to extend to the bulge
components of disk-dominated galaxies, and vice versa. We also find that, while
such massive galaxies at low z are bulge-dominated, at 1 < z < 2 they are
predominantly mixed bulge+disk systems, and by z > 2 they are mostly
disk-dominated. The majority of the disk-dominated galaxies are actively
forming stars, but this is also true for many of the bulge-dominated systems.
Interestingly, however, while most of the quiescent galaxies are
bulge-dominated, we find that a significant fraction (25-40%) of the most
quiescent galaxies have disk-dominated morphologies. Thus, while our results
show that the massive galaxy population is undergoing dramatic changes at this
crucial epoch, they also suggest that the physical mechanisms which quench
star-formation activity are not simply connected to those responsible for the
morphological transformation of massive galaxies into present-day giant
ellipticals.
