WHERE STARS FORM: INSIDE-OUT GROWTH and COHERENT STAR FORMATION from HST Hα MAPS of 3200 GALAXIES ACROSS the MAIN SEQUENCE at 0.7 < z < 1.5

We present Hα maps at 1 kpc spatial resolution for star-forming galaxies at z ∼ 1, made possible by the Wide Field Camera 3 grism on Hubble Space Telescope (HST). Employing this capability over all five 3D-HST/CANDELS fields provides a sample of 3200 galaxies enabling a division into subsamples based on stellar mass and star formation rate (SFR). By creating deep stacked Hα images, we reach surface brightness limits of 1 × 10^-18 erg s^-1 cm^-2 arcsec^-2, allowing us to map the distribution of ionized gas to ∼10 kpc for typical L∗ galaxies at this epoch. We find that the spatial extent of the Hα distribution increases with stellar mass as r_Hα = 1.5 (M_∗/10¹⁰ M_⊙)^0.23 kpc. The Hα emission is more extended than the stellar continuum emission, consistent with inside-out assembly of galactic disks. This effect grows stronger with mass as r_Hα/r_∗ = 1.1(M_∗/10¹⁰ M_⊙)^0.054. We map the Hα distribution as a function of SFR(IR+UV) and find evidence for "coherent star formation" across the SFR-M_∗ plane: above the main sequence (MS), Hα is enhanced at all radii; below the MS, Hα is depressed at all radii. This suggests that at all masses the physical processes driving the enhancement or suppression of star formation act throughout the disks of galaxies. At high masses (10^10.5 < M_∗/M_⊙ < 10¹¹), above the MS, Hα is particularly enhanced in the center, potentially building bulges and/or supermassive black holes. Below the MS, a strong central dip in the EW(Hα), as well as the inferred specific SFR, appears. Importantly, though, across the entirety of the SFR-M_∗ plane, the absolute SFR as traced by Hα is always centrally peaked, even in galaxies below the MS.