James Webb Space Telescope Feed Post

Harvard ADS: RUBIES Reveals a Massive Quiescent Galaxy at z=7.3

Paper abstract: We report the spectroscopic discovery of a massive quiescent galaxy at z_{\rm spec}=7.29\pm0.01, 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, 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\,\mum) shows that the galaxy formed a stellar mass of log(M_*/M_\odot)=10.23^{+0.04}_{-0.04} in a rapid ~ 100-200\,Myr burst of star formation at z~8-9, and ceased forming stars by z~8 resulting in \log \rm{sSFR/yr}^{-1}<-10. 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}/{\rm M_\odot\,kpc}^{-2})=10.85_{-0.12}^{+0.11} comparable to the 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\times larger than predicted from any model to date, indicating that quiescent galaxies have formed earlier than previously expected.