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

• Published in: Astrophysical journal. Letters Vol. 969; no. 1; p. L13
• Main Authors: Wang, Bingjie, Leja, Joel, de Graaff, Anna, Brammer, Gabriel B., Weibel, Andrea, van Dokkum, Pieter, Baggen, Josephine F. W., Suess, Katherine A., Greene, Jenny E., Bezanson, Rachel, Cleri, Nikko J., Hirschmann, Michaela, Labbé, Ivo, Matthee, Jorryt, McConachie, Ian, Naidu, Rohan P., Nelson, Erica, Oesch, Pascal A., Setton, David J., Williams, Christina C.
• Format: Journal Article
• Language: English
• Published: Austin The American Astronomical Society 01-07-2024; IOP Publishing
• Subjects: Active galactic nuclei; AGN host galaxies; Astronomical models; Balmer lines; Black holes; Cosmic dust; Dust; Emission; Emission lines; Emissions; Galaxies; Galaxy evolution; Galaxy formation; High-redshift galaxies; James Webb Space Telescope; Space telescopes; Spectral energy distribution; Spectroscopy; Spectrum analysis; Star & galaxy formation; Star formation; Stars & galaxies; Stellar populations
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/ad55f7

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 millions of years into the past in galaxies only 600–800 Myr after the big bang. Two of the three galaxies also show broad Balmer lines, with H β FWHM > 2500 km s −1 , suggesting that dust-reddened AGNs contribute to, or even dominate, the spectral energy distributions of these galaxies at λ rest ≳ 0.6 μ m. All three galaxies have relatively narrow [O iii ] lines, seemingly ruling out a high-mass interpretation if the lines arise in dynamically relaxed, inclined disks. Yet the inferred masses also remain highly uncertain. We model the high-quality spectra using Prospector to decompose the continuum into stellar and AGN components and explore limiting cases in stellar/AGN contribution. This produces a wide range of possible stellar masses, spanning M ⋆ ∼ 10 9 −10 11 M ⊙ . Nevertheless, all fits suggest a very early and rapid formation, most of which follow with a truncation in star formation. Potential origins and evolutionary tracks for these objects are discussed, from the cores of massive galaxies to low-mass galaxies with overmassive black holes. Intriguingly, we find all of these explanations to be incomplete; deeper and redder data are needed to understand the physics of these systems.