Excavating the Ruins: An Ancient z = 2.675 Galaxy Which Formed in the First 500 Myr

Abstract We present the analysis of an ancient galaxy at z = 2.675, which we dub “Eridu.” Simultaneously modeling the JWST/NIRSpec G140M and G235M spectra from the SMILES program and 0.4–25 μ m Hubble Space Telescope, JWST/NIRCam, and JWST/MIRI photometry from the JADES+SMILES photometric catalogs shows that Eridu is massive and quiescent, with stellar mass log ( M * / M ⊙ ) = 10.9 6 − 0.01 + 0.01 and average star formation rate 10, nearly 2 orders of magnitude more than the most massive current high-redshift observations. From Eridu’s spectrum, we infer [ Mg / Fe ] = + 0.6 5 − 0.19 + 0.20 , indicating its stellar population is extremely α -enhanced, consistent with the rapid formation timescale inferred from its SFH. We show that the inferred metallicity varies ∼0.4 dex between solar-scaled and α -enhanced models. As α -enhancement is expected in high- z quiescent galaxies, we speculate the common practice of spectral energy distribution modeling with solar-scaled abundance patterns could systematically affect the inferred ages of these objects. Eridu inhabits a massive protostructure that offers additional explanations for rapid mass assembly and quenching via environmental mechanisms, e.g., major mergers. Although its inferred formation is at odds with observations of the brightest z > 10 galaxies, future high-redshift galaxy formation studies and updated α -enhanced stellar models will unearth how Eridu and the first quiescent galaxies formed in the extremely early Universe.