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Confirmation of the Remarkable Compactness of Massive Quiescent Galaxies at z~2.3: Early-Type Galaxies Did not Form in a Simple Monolithic Collapse

Confirmation of the Remarkable Compactness of Massive Quiescent Galaxies at z~2.3: Early-Type Galaxies Did not Form in a Simple Monolithic Collapse

Publication date: 11 April 2008

Authors: van Dokkum, P.G. et al.

Journal: ApJL
Volume: 677
Issue: 1
Page: L5-L8
Year: 2008

Copyright: American Astronomical Society

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 x 1011 MSun, 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 H5 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.

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