Structural Evolution in Massive Galaxies at z ∼ 2

We present 0.″2 resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations at 870 μm in a stellar mass–selected sample of 85 massive ($M_{\star }>{10}^{11}{M}_{\odot }$) star-forming galaxies (SFGs) at $z=1.9\text{--}2.6$ in the CANDELS/3D-Hubble Space Telescope fields of UDS and GOODS-S. We measure the effective radius of the rest-frame far-infrared (FIR) emission for 62 massive SFGs. They are distributed over wide ranges of FIR size from $R_{e,FIR}=0.4kpc$ to $R_{e,FIR}=6kpc$. The effective radius of the FIR emission is smaller by a factor of ${2.3}_{-1.0}^{+1.9}$ than the effective radius of the optical emission and is smaller by a factor of ${1.9}_{-1.0}^{+1.9}$ than the half-mass radius. Taking into account potential extended components, the FIR size would change only by ∼10%. By combining the spatial distributions of the FIR and optical emission, we investigate how galaxies change the effective radius of the optical emission and the stellar mass within a radius of 1 kpc, $M_{1kpc}$. The compact starburst puts most of the massive SFGs on the mass–size relation for quiescent galaxies (QGs) at z ∼ 2 within 300 Myr if the current star formation activity and its spatial distribution are maintained. We also find that within 300 Myr, ∼38% of massive SFGs can reach the central mass of $M_{1kpc}={10}^{10.5}{M}_{\odot }$, which is around the boundary between massive SFGs and QGs. These results suggest an outside-in transformation scenario in which a dense core is formed at the center of a more extended disk, likely via dissipative in-disk inflows. Synchronized observations at ALMA 870 μm and James Webb Space Telescope 3–4 μm will explicitly verify this scenario.