COMPACT STARBURSTS IN z 3–6 SUBMILLIMETER GALAXIES REVEALED BY ALMA

We report the source size distribution, as measured by ALMA millimetric continuum imaging, of a sample of 13 AzTEC-selected submillimeter galaxies (SMGs) at zₚhoto ~ 3-6. Their infrared luminosities and star-formation rates (SFR) are LIR ~ 2-6 x 10¹2 Lₛun and ~ 200-600 Mₛun yr-1, respectively. The size of z ~ 3-6 SMGs ranges from 0". 10 to 0". 38 with a median of 0". 20+0". 03-0". 05 (FWHM), corresponding to a median circularized effective radius (Rc, e) of 0. 67+0. 13-0. 14 kpc, comparable to the typical size of the stellar component measured in compact quiescent galaxies at z ~ 2 (cQGs) --- R ~ 1 kpc. The median surface SFR density of our z ~ 3-6 SMGs is 100+42-26 Mₛun yr-1 kpc-2, comparable to that seen in local merger-driven (U) LIRGsrather than in extended disk galaxies at low and high redshifts. The discovery of compact starbursts in z >~ 3 SMGs strongly supports a massive galaxy formation scenario wherein z ~ 3-6 SMGs evolve into the compact stellar components of z ~ 2 cQGs. These cQGs are then thought to evolve into the most massive ellipticals in the local Universe, mostly via dry mergers. Our results thus suggest that z >~ 3 SMGs are the likely progenitors of massive local ellipticals, via cQGs, meaning that we can now trace the evolutionary path of the most massive galaxies over a period encompassing ~ 90% of the age of the Universe.