RUBIES Reveals a Massive Quiescent Galaxy at z=7.3

Abstract

We report the spectroscopic discovery of a massive quiescent galaxy at $z_{\mathrm{spec}}=7.29\pm 0.01$, just $\sim 700$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 $0.9-18\mu m$) shows that the galaxy formed a stellar mass of log$(M_{\ast }/M_{\odot })=10.23_{-0.04}^{+0.04}$ before $z\sim 8$, 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 $209_{-24}^{+33}\mathrm{pc}$, which implies a high stellar mass surface density within the effective radius of $\log ({\Sigma }_{\ast ,\mathrm{e}}/M_{\odot }kp{c}^{-2})=10.85_{-0.12}^{+0.11}$ comparable to the highest densities measured in quiescent galaxies at $z\sim 2-5$. 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 $z\sim 7$ is $>100\times$ larger than predicted from any model to date, indicating that quiescent galaxies have formed earlier than previously expected.

22 pages, 8 figures, accepted for publication in ApJ

Cited by in corpus (1)

• JWST/NIRSpec spectroscopy of intermediate-mass quiescent galaxies at $z\sim 3-4$