2025ApJ...983...11W

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 μm) shows that the galaxy formed a stellar mass of log$({M}_{* }/{M}_{\odot })=10.2{3}_{-0.04}^{+0.04}$ before z ∼ 8 and ceased forming stars 50-100 Myr prior to the time of observation, resulting in $\mathrm{log}({{\rm{sSFR/Gyr}}}^{-1})\lt -1$. We measure a small physical size of $20{9}_{-24}^{+33}\,{\rm{pc}}$, which implies a high stellar-mass surface density within the effective radius of $\mathrm{log}({{\rm{\Sigma }}}_{* ,{\rm{e}}}/{M}_{\odot }\,{{\rm{kpc}}}^{-2})=10.8{5}_{-0.12}^{+0.11}$, 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.