Morphological evolution of galaxies from ultradeep HST WFC3 imaging: the
  Hubble sequence at z~2
  We use ultradeep HST WFC3/IR imaging of the HUDF to investigate the
rest-frame optical morphologies of a mass-selected sample of galaxies at z~2.
We find a large variety of galaxy morphologies, ranging from large, blue,
disk-like galaxies to compact, red, early-type galaxies. We derive rest-frame
u-g color profiles for these galaxies and show that most z~2 galaxies in our
sample have negative color gradients such that their cores are red. Although
these color gradients may partly be caused by radial variations in dust
content, they point to the existence of older stellar populations in the
centers of z~2 galaxies. This result is consistent with an "inside-out"
scenario of galaxy growth. We find that the median color gradient is fairly
constant with redshift: [Delta(u-g_{rest})/Delta(log r)]_{median} = -0.46,
-0.44 and -0.49 for z~2, z~1 and z=0, respectively. Using structural parameters
derived from surface brightness profiles we confirm that at z~2 galaxy
morphology correlates well with specific star formation rate. At the same mass,
star forming galaxies have larger effective radii, bluer rest-frame u-g colors
and lower Sersic indices than quiescent galaxies. These correlations are very
similar to those at lower redshift, suggesting that the relations that give
rise to the Hubble sequence at z=0 are already in place for massive galaxies at
this early epoch.
