Confirmation of the remarkable compactness of massive quiescent galaxies at z ∼ 2.3:: Early-type galaxies did not form in a simple monolithic collapse

Using deep near-infrared spectroscopy, Kriek et al. found that similar to 45% of massive galaxies at z similar to 2.3 have evolved stellar populations and little or no ongoing star formation. Here we determine the sizes of these quiescent galaxies using deep, high-resolution images obtained with HST/NIC2 and laser guide star (LGS)-assisted Keck/adaptive optics (AO). Considering that their median stellar mass is 1.7 x 10(11) M-circle dot, the galaxies are remarkably small, with a median effective radius r(e) = 0.9 kpc. Galaxies of similar mass in the nearby universe have sizes of approximate to 5 kpc and average stellar densities that are 2 orders of magnitude lower than the z similar to 2.3 galaxies. These results extend earlier work at z similar to 1.5 and confirm previous studies at z > 2 that lacked spectroscopic redshifts and imaging of sufficient resolution to resolve the galaxies. Our findings demonstrate that fully assembled early-type galaxies make up at most similar to 10% of the population of K-selected quiescent galaxies at z similar to 2.3, effectively ruling out simple monolithic models for their formation. The galaxies must evolve significantly after z similar to 2.3, through dry mergers or other processes, consistent with predictions from hierarchical models.