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 have evolved z ∼ 2.3 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 , the galaxies are remarkably small, with 11 1.7 # 10 M, a median effective radius kpc. Galaxies of similar mass in the nearby universe have sizes of ≈5 kpc and r p 0.9 e average stellar densities that are 2 orders of magnitude lower than the galaxies. These results extend earlier z ∼ 2.3 work at and confirm previous studies at that lacked spectroscopic redshifts and imaging of sufficient z ∼ 1.5 z 1 2 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 , effectively ruling out simple monolithic z ∼ 2.3 models for their formation. The galaxies must evolve significantly after , through dry mergers or other z ∼ 2.3 processes, consistent with predictions from hierarchical models. Subject headings: cosmology: observations — galaxies: evolution — galaxies: formation