  We analyze the resolved stellar populations of 473 massive star-forming
galaxies at 0.7 < z < 1.5, with multi-wavelength broad-band imaging from
CANDELS and Halpha surface brightness profiles at the same kiloparsec
resolution from 3D-HST. Together, this unique data set sheds light on how the
assembled stellar mass is distributed within galaxies, and where new stars are
being formed. We find the Halpha morphologies to resemble more closely those
observed in the ACS I band than in the WFC3 H band, especially for the larger
systems. We next derive a novel prescription for Halpha dust corrections, which
accounts for extra extinction towards HII regions. The prescription leads to
consistent SFR estimates and reproduces the observed relation between the
Halpha/UV luminosity ratio and visual extinction, both on a pixel-by-pixel and
on a galaxy-integrated level. We find the surface density of star formation to
correlate with the surface density of assembled stellar mass for spatially
resolved regions within galaxies, akin to the so-called 'main sequence of star
formation' established on a galaxy-integrated level. Deviations from this
relation towards lower equivalent widths are found in the inner regions of
galaxies. Clumps and spiral features, on the other hand, are associated with
enhanced Halpha equivalent widths, bluer colors, and higher specific star
formation rates compared to the underlying disk. Their Halpha/UV luminosity
ratio is lower than that of the underlying disk, suggesting the ACS clump
selection preferentially picks up those regions of elevated star formation
activity that are the least obscured by dust. Our analysis emphasizes that
monochromatic studies of galaxy structure can be severely limited by
mass-to-light ratio variations due to dust and spatially inhomogeneous star
formation histories.
