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We investigate the build-up of galaxies at z~1 using maps of Halpha andstellar continuum emission for a sample of 57 galaxies with rest-frame Halphaequivalent widths >100 Angstroms in the 3D-HST grism survey. We find that theHalpha emission broadly follows the rest-frame R-band light but that it istypically somewhat more extended and clumpy. We quantify the spatialdistribution with the half-light radius. The median Halpha effective radiusr_e(Halpha) is 4.2+-0.1 kpc but the sizes span a large range, from compactobjects 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, astraced by Halpha, typically occurs out to larger radii than the rest-frameR-band stellar continuum; galaxies are growing their radii and building up fromthe inside out. This effect appears to be somewhat more pronounced for thelargest galaxies. Using the measured Halpha sizes, we derive star formationrate 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 smallestgalaxies, implying a large range in physical conditions in rapidly star-formingz~1 galaxies. Finally, we infer that all galaxies in the sample have very highgas mass fractions and stellar mass doubling times < 500 Myr. Although otherexplanations are also possible, a straightforward interpretation is that we aresimultaneously witnessing the rapid formation of compact bulges and large disksat z~1.

the astrophysical journal. letters

Spatially resolved Halpha maps and sizes of 57 strongly star-forming  galaxies at z~1 from 3D-HST: evidence for rapid inside-out assembly of disk  galaxies