BULGE-FORMING GALAXIES with AN EXTENDED ROTATING DISK at z ∼ 2

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 _o) > 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, , and is comparable with optical sizes of massive quiescent galaxies at similar redshifts. As they have an exponential disk with Sérsic index of 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 M _o yr^-1 kpc^-2, the intense circumnuclear starbursts can rapidly build up a central bulge with ΣM _{∗,1 kpc} > 10¹⁰ M _o 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.