2015ApJ...810..133I

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 x 10¹²L_☉ and ∼200-600 M_☉/yr, respectively. The sizes of these SMGs range from 0".10 to 0".38, with a median of 0".20_0"0.5^+0"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⁺⁴²M_☉/yr/kpc2, 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.