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 1 using maps of Hα and stellar continuum emission for a sample of 57 galaxies with rest-frame Hα equivalent widths >100 Å in the 3D-HST grism survey. We find that the Hα 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 Hα effective radius r_e (Hα) is 4.2 ± 0.1kpc but the sizes span a large range, from compact objects with r_e (Hα) 1.0kpc to extended disks with r_e (Hα) 15kpc. Comparing Hα sizes to continuum sizes, we find <r_e (Hα)/r_e (R) > =1.3 ± 0.1 for the full sample. That is, star formation, as traced by Hα, 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 Hα sizes, we derive star formation rate surface densities, Σ_SFR. We find that Σ_SFR ranges from 0.05 M yr^-1kpc^-2 for the largest galaxies to 5 M yr^-1kpc^-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 <500Myr. 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.