An Extremely Massive Quiescent Galaxy at $z=3.493$: Evidence of Insufficiently Rapid Quenching Mechanisms in Theoretical Models

TL;DR

This paper reports the discovery of the most massive quiescent galaxy at high redshift, revealing that current models underestimate the rapidity of quenching mechanisms needed to form such galaxies early in the universe.

Contribution

It provides spectroscopic confirmation of an extremely massive quiescent galaxy at z=3.493, challenging existing models of galaxy formation and quenching.

Findings

Galaxy has no significant ongoing star formation.

Galaxy's stellar mass is nearly three times higher than previous high-redshift examples.

Current models do not reproduce such massive, quiescent galaxies at this epoch.

Abstract

We present spectra of the most massive quiescent galaxy yet discovered at $z>3$, spectroscopically confirmed via the detection of Balmer absorption features in the $H-$ and $K-$bands of Keck/MOSFIRE. The spectra confirm a galaxy with no significant ongoing star formation, consistent with the lack of rest-frame UV flux and overall photometric spectral energy distribution. With a stellar mass of $3.1^{+0.1}_{-0.2} \times 10^{11} ~\rm{M}_\odot$ at $z = 3.493$, this galaxy is nearly three times more massive than the highest redshift spectroscopically confirmed absorption-line identified galaxy known. The star-formation history of this quiescent galaxy implies that it formed $>1000 ~\rm{M}_\odot$/yr for almost 0.5 Gyr beginning at $z\sim7.2$, strongly suggestive that it is the descendant of massive dusty star-forming galaxies at $5<z<7$ recently observed with ALMA. While galaxies with similarly extreme stellar masses are reproduced in some simulations at early times, such a lack of ongoing star formation is not seen there. This suggests the need for a more rapid quenching process than is currently prescribed, challenging our current understanding of how ultra-massive galaxies form and evolve in the early Universe.