RUBIES spectroscopically confirms the high number density of quiescent galaxies from $\mathbf{2<z<5}$

TL;DR

This study uses JWST NIRSpec spectra from RUBIES to robustly measure the number density of massive quiescent galaxies between redshifts 2 and 5, revealing their surprising abundance and challenging current galaxy formation models.

Contribution

It provides the most complete spectroscopic estimates of massive quiescent galaxy densities at high redshift and compares these with cosmological simulations to test galaxy formation theories.

Findings

Massive quiescent galaxies are more common at high redshift than previously thought.

Most galaxy formation simulations underpredict the number of early quiescent galaxies.

The results challenge current models of feedback and early galaxy formation channels.

Abstract

We present the number density of massive ($ \mathrm{ log (M_{*}/M_{\odot}) > 10.3} $) quiescent galaxies at $2<z<5$ using JWST NIRSpec PRISM spectra. This work relies on spectra from RUBIES, which provides excellent data quality and an unparalleled, well-defined targeting strategy to robustly infer physical properties and number densities. We identify quiescent galaxy candidates within RUBIES through principal component analysis and construct a final sample using star formation histories derived from spectro-photometric fitting of the NIRSpec PRISM spectra and NIRCam photometry. By inverting the RUBIES selection function, we correct for survey incompleteness and calculate the number density of massive quiescent galaxies at these redshifts, providing the most complete spectroscopic estimates prior to cosmic noon to date. We find that early massive quiescent galaxies are surprisingly common ($\gtrsim 10^{-5}$ Mpc$^{-3}$ by $4<z<5$), which is consistent with previous studies based on JWST photometry alone and/or in smaller survey areas. We compare our number densities with predictions from six state-of-the-art cosmological galaxy formation simulations. At $z>3$, most simulations fail to produce enough massive quiescent galaxies, suggesting the treatment of feedback and/or the channels for early efficient formation are incomplete in most galaxy evolution models.