The Bulge-Disk Decomposed Evolution of Massive Galaxies at 1<z<3 in
  CANDELS
  We present the results of a new and improved study of the morphological and
spectral evolution of massive galaxies over the redshift range 1<z<3. Our
analysis is based on a bulge-disk decomposition of 396 galaxies with
Mstar>10^11 Msolar from the CANDELS WFC3/IR imaging within the COSMOS and
UKIDSS UDS survey fields. We find that, by modelling the H(160) image of each
galaxy with a combination of a de Vaucouleurs bulge (Sersic index n=4) and an
exponential disk (n=1), we can then lock all derived morphological parameters
for the bulge and disk components, and successfully reproduce the
shorter-wavelength J(125), i(814), v(606) HST images simply by floating the
magnitudes of the two components. This then yields sub-divided 4-band HST
photometry for the bulge and disk components which, with no additional priors,
is well described by spectrophotometric models of galaxy evolution. Armed with
this information we are able to properly determine the masses and
star-formation rates for the bulge and disk components, and find that: i) from
z=3 to z=1 the galaxies move from disk-dominated to increasingly
bulge-dominated, but very few galaxies are pure bulges/ellipticals by z=1; ii)
while most passive galaxies are bulge-dominated, and most star-forming galaxies
disk-dominated, 18+/-5% of passive galaxies are disk-dominated, and 11+/-3% of
star-forming galaxies are bulge-dominated, a result which needs to be explained
by any model purporting to connect star-formation quenching with morphological
transformations; iii) there exists a small but significant population of pure
passive disks, which are generally flatter than their star-forming counterparts
(whose axial ratio distribution peaks at b/a~0.7); iv) flatter/larger disks
re-emerge at the highest star-formation rates, consistent with recent studies
of sub-mm galaxies, and with the concept of a maximum surface-density for
star-formation activity.
