SPATIALLY RESOLVED Hα MAPS AND SIZES OF 57 STRONGLY STAR-FORMING GALAXIES AT z ∼ 1 FROM 3D-HST: EVIDENCE FOR RAPID INSIDE-OUT ASSEMBLY OF DISK GALAXIES

We investigate the buildup of galaxies at z similar to 1 using maps of H alpha and stellar continuum emission for a sample of 57 galaxies with rest-frame H alpha equivalent widths > 100 angstrom in the 3D-HST grism survey. We find that the H alpha 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 H alpha effective radius r(e)(H alpha) is 4.2 +/- 0.1 kpc but the sizes span a large range, from compact objects with r(e)(H alpha) similar to 1.0 kpc to extended disks with r(e)(H alpha) similar to 15 kpc. Comparing Ha sizes to continuum sizes, we find < r(e)(H alpha)/r(e)(R)> = 1.3 +/- 0.1 for the full sample. That is, star formation, as traced by H alpha, 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 Ha sizes, we derive star formation rate surface densities, Sigma(SFR). We find that Sigma(SFR) ranges from similar to 0.05 M-circle dot yr(-1) kpc(-2) for the largest galaxies to similar to 5 M-circle dot yr(-1) kpc(-2) for the smallest galaxies, implying a large range in physical conditions in rapidly star-forming z similar to 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 similar to 1.