RUBIES: Evolved Stellar Populations with Extended Formation Histories at z ∼ 7–8 in Candidate Massive Galaxies Identified with JWST/NIRSpec

Abstract The identification of red, apparently massive galaxies at z > 7 in early James Webb Space Telescope (JWST) photometry suggests a strongly accelerated time line compared to standard models of galaxy growth. A major uncertainty in the interpretation is whether the red colors are caused by evolved stellar populations, dust, or other effects such as emission lines or active galactic nuclei (AGNs). Here we show that three of the massive galaxy candidates at z = 6.7–8.4 have prominent Balmer breaks in JWST/NIRSpec spectroscopy from the RUBIES program. The Balmer breaks demonstrate unambiguously that stellar emission dominates at λ rest = 0.4 μ m and require formation histories extending hundreds of…

• NA
  National Aeronautics and Space Administration

  Awards: 5-03127, NAS 5-03127, JWST-GO-04233.009-A
• ST
  Space Telescope Science Institute

  Awards: 5-03127, NAS 5-03127
• NR
  National Research Foundation
• DG
  Danmarks Grundforskningsfond

  Award: No. 140
• IF
  Institute for Computational and Data Sciences, Pennsylvania State University