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 $0.4-25\ \mu\mathrm{m}$ HST, JWST/NIRCam, and JWST/MIRI photometry from the the JADES+SMILES photometric catalogs shows that Eridu is massive and quiescent with stellar mass $$\log(M_<em>/\mathrm{M_\odot})=10.96<sup>{+0.01}_{-0.01}$$ and average star formation rate $&lt;1\ \mathrm{M_\odot\ yr<sup>{-1}}$ over the last 100 Myr. Star formation histories inferred from various models produce disconcertingly early and fast formation within $\sim300$ Myr of the Big Bang and quenching 2 Gyr prior to observation ($z\sim10$). This stellar mass assembly implies that the progenitor of Eridu had $M_</em>\approx10^{11}\ \mathrm{M_\odot}$ at $z&gt;10$, nearly two orders of magnitude more than the most massive current high redshift observations. From Eridu's spectrum we infer $\mathrm{[Mg/Fe]} =+0.65<sup>{+0.20}_{-0.19}$, indicating its stellar population is extremely $\alpha$-enhanced, which is consistent with the rapid formation timescale inferred from its star formation history. Eridu inhabits a massive protostructure which offers additional explanations for rapid mass assembly and quenching via environmental mechanisms, e.g. major mergers. Though its inferred formation is at odds with observations of the brightest cosmic dawn galaxies, we anticipate that future high-redshift galaxy formation models and sophisticated stellar population modeling codes will unearth how Eridu formed at the dawn of time.