ALMA reveals rapid formation of a dense core for massive disks at z~2

At z~2, massive quiescent galaxies are extremely compact with a dense core while the majority of star-forming galaxies have a more extended stellar disk. However, it remains unknown how the dense core is formed and how galaxies are transformed. We have made 1kpc-resolution ALMA observations at 870 um for 25 massive galaxies on the star formation main sequence at z~2, and compared the spatial distribution of the rest-frame far-infrared emission with stellar mass maps derived from spatially resolved stellar population modeling. The high-resolution ALMA data reveal that the dust continuum emission is mostly radiated from single region close to the galaxy center and its half light radius is more compact by a factor of 2-3 than the rest-frame optical light and stellar component. Given extremely high SFR surface densities measured within a central 1 kpc, the compact starburst can build up a dense core in only a few hundred Myr. We may be witnessing one of evolutionary pathways from extended star-forming disks to compact galaxies