RUBIES Reveals a Massive Quiescent Galaxy at z = 7.3

Abstract We report the spectroscopic discovery of a massive quiescent galaxy at z spec = 7.29 ± 0.01, just ∼700 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, with 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 μ m) shows that the galaxy formed a stellar mass of log ( M * / M ⊙ ) = 10.2 3 − 0.04 + 0.04 before z ∼ 8 and ceased forming stars 50–100 Myr prior to the time of observation, resulting in log ( sSFR/Gyr − 1 ) − 1 . We measure a small physical size of 20 9 − 24 + 33 pc , which implies a high stellar-mass surface density within the effective radius of log ( Σ * , e / M ⊙ kpc − 2 ) = 10.8 5 − 0.12 + 0.11 , comparable to the highest densities measured in quiescent galaxies at z ∼ 2–5. 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 ∼ 7 is >100 × larger than predicted from any model to date, indicating that quiescent galaxies have formed earlier than previously expected.