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arXiv · 0802.4094

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

Pieter van Dokkum·Marijn Franx·Mariska Kriek·Bradford Holden·Garth Illingworth·Daniel Magee·Rychard Bouwens·Danilo Marchesini·Ryan Quadri·Greg Rudnick·Edward Taylor·Sune Toft
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Abstract

Using deep near-infrared spectroscopy Kriek et al. (2006) 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-assisted Keck/AO. Considering that their median stellar mass is 1.7x10^11 Solar masses the galaxies are remarkably small, with a median effective radius of 0.9 kpc. Galaxies of similar mass in the nearby Universe have sizes of ~5 kpc and average stellar densities which are two 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 which 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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Pieter van Dokkum, Marijn Franx, Mariska Kriek, Bradford Holden, Garth Illingworth, Daniel Magee, Rychard Bouwens, Danilo Marchesini, Ryan Quadri, Greg Rudnick, Edward Taylor, Sune Toft. 2008-02-27. Confirmation of the remarkable compactness of massive quiescent galaxies at z~2.3: early-type galaxies did not form in a simple monolithic collapse. https://doi.org/10.1086/587874

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