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.