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 ~45% of massive galaxies at z ~ 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 × 1011 M☉, the galaxies are remarkably small, with a median effective radius re = 0.9 kpc. Galaxies of similar mass in the nearby universe have sizes of ≈5 kpc and average stellar densities that are 2 orders of magnitude lower than the z ~ 2.3 galaxies. These results extend earlier work at z ~ 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 ~10% of the population of K-selected quiescent galaxies at z ~ 2.3, effectively ruling out simple monolithic models for their formation. The galaxies must evolve significantly after z ~ 2.3, through dry mergers or other processes, consistent with predictions from hierarchical models.