  We investigate the build-up of galaxies at z~1 using maps of Halpha and
stellar continuum emission for a sample of 57 galaxies with rest-frame Halpha
equivalent widths >100 Angstroms in the 3D-HST grism survey. We find that the
Halpha emission broadly follows the rest-frame R-band light but that it is
typically somewhat more extended and clumpy. We quantify the spatial
distribution with the half-light radius. The median Halpha effective radius
r_e(Halpha) is 4.2+-0.1 kpc but the sizes span a large range, from compact
objects with r_e(Halpha) ~ 1.0 kpc to extended disks with r_e(Halpha) ~ 15 kpc.
Comparing Halpha sizes to continuum sizes, we find
<r_e(Halpha)/r_e(R)>=1.3+-0.1 for the full sample. That is, star formation, as
traced by Halpha, typically occurs out to larger radii than the rest-frame
R-band stellar continuum; galaxies are growing their radii and building up from
the inside out. This effect appears to be somewhat more pronounced for the
largest galaxies. Using the measured Halpha sizes, we derive star formation
rate surface densities. We find that they range from ~0.05 Msun yr^{-1}
kpc^{-2} for the largest galaxies to ~5 Msun yr^{-1} kpc^{-2} for the smallest
galaxies, implying a large range in physical conditions in rapidly star-forming
z~1 galaxies. Finally, we infer that all galaxies in the sample have very high
gas mass fractions and stellar mass doubling times < 500 Myr. Although other
explanations are also possible, a straightforward interpretation is that we are
simultaneously witnessing the rapid formation of compact bulges and large disks
at z~1.
