RUBIES Reveals a Massive Quiescent Galaxy at <i>z</i> = 7.3
Andrea Weibel, Anna de Graaff, David J. Setton, Tim B. Miller, Pascal A. Oesch, Gabriel Brammer, Claudia del P. Lagos, Katherine E. Whitaker, Christina C. Williams, Josephine F. W. Baggen, Rachel Bezanson, Leindert Boogaard, Nikko J. Cleri, Jenny E. Greene, Michaela Hirschmann, Raphael E. Hviding, A. Kuruvanthodi, Ivo Labbé, Joel Leja, Michael V. Maseda, Jorryt Matthee, Ian McConachie, Rohan P. Naidu, Guido Roberts-Borsani, D. Schaerer, Katherine A. Suess, Francesco Valentino, Pieter van Dokkum, Bingjie Wang
Abstract
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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>*</mml:mo> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mo>=</mml:mo> <mml:mn>10.2</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.04</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.04</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> before z ∼ 8 and ceased forming stars 50–100 Myr prior to the time of observation, resulting in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">sSFR/Gyr</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:mo stretchy="false">)</mml:mo> <mml:mo><</mml:mo> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:math> . We measure a small physical size of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>20</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>9</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>24</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>33</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.25em"/> <mml:mi mathvariant="normal">pc</mml:mi> </mml:math> , which implies a high stellar-mass surface density within the effective radius of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mo mathvariant="normal">Σ</mml:mo> </mml:mrow> <mml:mrow> <mml:mo>*</mml:mo> <mml:mo>,</mml:mo> <mml:mi mathvariant="normal">e</mml:mi> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">kpc</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> <mml:mo stretchy="false">)</mml:mo> <mml:mo>=</mml:mo> <mml:mn>10.8</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>5</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.12</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.11</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , 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.