Rapid formation of a central bulge in massive galaxies at z~2: from MAHALO-Subaru to GRACIAS-ALMA

Kenichi Tadaki

At z~2, the majority of star-forming galaxies have an extended disk while massive quiescent galaxies are extremely compact with a dense core. However, the link between extended star-forming disks and compact bulge-dominated quiescent galaxies is still missing. In `GRACIAS-ALMA' project, we have made 1kpc-resolution ALMA observations at 870 um for 25 massive field galaxies on the star formation main sequence at z~2, which are originally identified by our narrow-band survey with the MOIRCS in `MAHALO-Subaru' project. We find many of them to be associated with extremely compact dust emission, which is more than a factor of 2 smaller than their rest-optical sizes. As they have an exponential disk 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, the intense circumnuclear starbursts can rapidly build up a central bulge in several hundred Myr, i.e. by z~2.0. I also report a current status of GRACIAS-ALMA project targeting SFGs in high-density environments.