RUBIES Reveals a Massive Quiescent Galaxy at z=7.3

TL;DR

This paper reports the discovery of a massive, quiescent galaxy at redshift 7.3, revealing that such galaxies formed earlier and are more common than current models predict, with implications for galaxy formation theories.

Contribution

It presents the first spectroscopic confirmation of a massive quiescent galaxy at z>7, showing its properties and challenging existing galaxy formation models.

Findings

The galaxy has a stellar mass of about 1.7×10^10 solar masses.

It ceased star formation 50-100 million years before observation.

The galaxy's core density resembles local massive ellipticals.

Abstract

We report the spectroscopic discovery of a massive quiescent galaxy at $z_{\rm spec}=7.29\pm0.01$, just $\sim700\,$Myr after the Big Bang. RUBIES-UDS-QG-z7 was selected from public JWST/NIRCam and MIRI imaging from the PRIMER survey and observed with JWST/NIRSpec as part of RUBIES. The NIRSpec/PRISM spectrum reveals one of the strongest Balmer breaks observed thus far at $z>6$, no emission lines, but tentative Balmer and Ca absorption features, as well as a Lyman break. Simultaneous modeling of the NIRSpec/PRISM spectrum and NIRCam and MIRI photometry (spanning $0.9-18\,\mu m$) shows that the galaxy formed a stellar mass of log$(M_*/M_\odot)=10.23^{+0.04}_{-0.04}$ before $z\sim8$, and ceased forming stars $50-100\,$Myr prior to the time of observation, resulting in $\log(\rm{sSFR/Gyr}^{-1})<-1$. We measure a small physical size of $209_{-24}^{+33}\,{\rm pc}$, which implies a high stellar mass surface density within the effective radius of $\log(\Sigma_{*,\rm e}/M_\odot\,kpc^{-2})=10.85_{-0.12}^{+0.11}$ comparable to the highest densities measured in quiescent galaxies at $z\sim2-5$. The 3D stellar mass density profile of RUBIES-UDS-QG-z7 is remarkably similar to the central densities of local massive ellipticals, suggesting that at least some of their cores may have already been in place at $z>7$. The discovery of RUBIES-UDS-QG-z7 has strong implications for galaxy formation models: the estimated number density of quiescent galaxies at $z\sim7$ is $>100\times$ larger than predicted from any model to date, indicating that quiescent galaxies have formed earlier than previously expected.