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 z(phot) similar to 3-6. Their infrared luminosities and star formation rates (SFRs) are L-IR similar to, 2-6 x 10(12) L-circle dot and similar to 200-600 M-circle dot yr(-1), respectively. The sizes of these SMGs range from 0 ''.10 to 0 ''.38, with a median of 0 ''.20(-0 '' 05)(+0 '' 03) (FWHM), corresponding to a median circularized effective radius (R-c,R-e) of 0.67(-0.14)(+0.13) kpc, comparable to the typical size of the stellar component measured in compact quiescent galaxies at, Z similar to 2 (cQGs)-R-e similar to 1 kpc. The median surface SFR density of our SMGs is 100(-26)(+42) M-circle dot yr(-1) kpc(-2) -1 comparable to that seen in local merger-driven (U)LIRGs rather than in extended disk galaxies at low and high redshifts. The discovery of compact starbursts in z greater than or similar to 3 SMGs strongly supports a massive galaxy formation scenario wherein z similar to 3-6 SMGs evolve into the compact stellar components of z similar to 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 greater than or similar to 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 similar to 90% of the age of the universe.