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

TL;DR

This study confirms that massive quiescent galaxies at z~2.3 are extremely compact compared to local counterparts, challenging simple formation models and supporting hierarchical evolution.

Contribution

It provides high-resolution size measurements of spectroscopically confirmed quiescent galaxies at z~2.3, demonstrating their compactness and implications for galaxy formation theories.

Findings

Galaxies have median effective radius of 0.9 kpc.

These galaxies are two orders of magnitude denser than local counterparts.

Fully assembled early-type galaxies are at most 10% of the quiescent population at z~2.3.

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.