Abstract

We present 0."2-resolution Atacama Large Millimeter/submillimeter Array observations at 870 mu m for 25 H alpha-seleced star-forming galaxies around the main sequence at z = 2.2-2.5. We detect significant 870 mu m 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-circle dot) > 11. We find nine galaxies to be associated with extremely compact dust emission with R-1/2,R-870 mu m 1.5 kpc, which is more than a factor of 2 smaller than their rest-optical sizes, R-1/2,R-1.6 mu m > = 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(1.6 mu m)> = 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 kpc >= 40 M-circle dot yr (1) kpc (2), the intense circumnuclear starbursts can rapidly build up a central bulge with Sigma M-*,M-1 kpc > 10(10) M-circle dot kpc(-2) in several hundred megayears, i.e., by z similar to 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.