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 ∼ 3 ?> –6. Their infrared luminosities and star formation rates (SFRs) are L IR ∼ ?> 2– 6 × 10 12 ?> L ⊙ ?> and ∼200–600 M ⊙ ?> yr−1, respectively. The sizes of these SMGs range from 0.″10 to 0.″38, with a median of 0.″20 − 0 &farcs; 05 + 0 &farcs; 03 ?> (FWHM), corresponding to a median circularized effective radius ( R c , 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 ∼ 2 ?> (cQGs)— R e ∼ 1 ?> kpc. The median surface SFR density of our SMGs is 100 − 26 + 42 ?> M ⊙ ?> yr−1 kpc−2, 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 ≳ 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.