Excavating The Ruins: an Ancient $z=2.675$ Galaxy Which Formed in the First 500 Myr

TL;DR

This paper analyzes a massive, quiescent galaxy at redshift 2.675, revealing it formed rapidly within the first 500 million years of the universe, challenging current galaxy formation models.

Contribution

It presents detailed spectral and photometric analysis of Eridu, an ancient galaxy, showing its rapid formation and quenching in the early universe, with implications for high-redshift galaxy evolution.

Findings

Eridu is massive and quiescent with stellar mass ~10^11 M_sun at z>10.

It exhibits extreme alpha-enhancement, indicating rapid star formation.

Its formation timeline conflicts with existing cosmic dawn galaxy observations.

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 $\log(M_*/\mathrm{M_\odot})=10.96^{+0.01}_{-0.01}$ and average star formation rate $<1\ \mathrm{M_\odot\ yr^{-1}}$ 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 $M_*\approx10^{11}\ \mathrm{M_\odot}$ at $z>10$, nearly two orders of magnitude more than the most massive current high redshift observations. From Eridu's spectrum we infer $\mathrm{[Mg/Fe]} =+0.65^{+0.20}_{-0.19}$, 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.