2017ApJ...834..135T

We present 0.''2-resolution Atacama Large Millimeter/submillimeter Array observations at 870 µm for 25 Hα-seleced star-forming galaxies around the main sequence at z = 2.2-2.5. We detect significant 870 µm continuum emission in 16 (64%) of these galaxies. The high-resolution maps reveal that the dust emission is mostly radiated from a single region close to the galaxy center. Exploiting the visibility data taken over a wide uv distance range, we measure the half-light radii of the rest-frame far-infrared emission for the best sample of 12 massive galaxies with log(M_*/M_☉) > 11. We find nine galaxies to be associated with extremely compact dust emission with R_1/2,870 µm_ < 1.5 kpc, which is more than a factor of 2 smaller than their rest-optical sizes, <R_1/2,1.6µm_>=3.2 kpc, and is comparable with optical sizes of massive quiescent galaxies at similar redshifts. As they have an exponential disk with Sersic index of <n_1.6µm_>=1.2 in the rest-optical, they are likely to be in the transition phase from extended disks to compact spheroids. Given their high star formation rate surface densities within the central 1 kpc of <ΣSFR_1kpc>=40M_☉ yr^–1 kpc^–2, the intense circumnuclear starbursts can rapidly build up a central bulge with ΣM_*,1 kpc_ > 10¹⁰ M_☉ kpc^–2 in several hundred megayears, i.e., by z ∼ 2. Moreover, ionized gas kinematics reveal that they are rotation supported with an angular momentum as large as that of typical star-forming galaxies at z = 1-3. Our results suggest that bulges are commonly formed in extended rotating disks by internal processes, not involving major mergers.