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Using deep near-infrared spectroscopy Kriek et al. (2006) found that ~45% ofmassive galaxies at z~2.3 have evolved stellar populations and little or noongoing star formation. Here we determine the sizes of these quiescent galaxiesusing deep, high-resolution images obtained with HST/NIC2 and laser guidestar-assisted Keck/AO. Considering that their median stellar mass is 1.7x10^11Solar masses the galaxies are remarkably small, with a median effective radiusof 0.9 kpc. Galaxies of similar mass in the nearby Universe have sizes of ~5kpc and average stellar densities which are two orders of magnitude lower thanthe z~2.3 galaxies. These results extend earlier work at z~1.5 and confirmprevious studies at z>2 which lacked spectroscopic redshifts and imaging ofsufficient resolution to resolve the galaxies. Our findings demonstrate thatfully assembled early-type galaxies make up at most ~10% of the population ofK-selected quiescent galaxies at z~2.3, effectively ruling out simplemonolithic models for their formation. The galaxies must evolve significantlyafter z~2.3, through dry mergers or other processes, consistent withpredictions from hierarchical models.

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