Spectroscopic Confirmation of an Ultra-Massive Galaxy in a Protocluster at z ∼ 4.9

We present spectroscopic confirmation of an ultra-massive galaxy (UMG) with log(M_⋆/M_⊙) = 10.98 ± 0.09 at z_spec = 4.8947 in the Extended Groth Strip (EGS), based on deep observations of Lyα emission with Keck/DEIMOS. The ultra-massive galaxy (UMG-28740) is the most massive member in one of the most significant overdensities in the EGS, with four additional photometric members with log(M_⋆/M_⊙) > 10.5 within R_proj∼ 1 cMpc. Spectral energy distribution (SED) fitting using a large suite of star formation histories and two sets of high-quality photometry from ground- and space-based facilities consistently estimates the mass of this object to be log(M_⋆/M_⊙) ∼ 11 with a small standard deviation between measurements (σ = 0.09). While the best-fit SED models agree on stellar mass, we find discrepancies in the estimated star formation rate for UMG-28740, resulting in either a star-forming or quiescent system. 𝐽𝑊𝑆𝑇/NIRCam photometry of UMG-28740 strongly favors a quiescent scenario, demonstrating the need for high-quality mid-IR observations. Assuming the galaxy to be quiescent, UMG-28740 formed the bulk of its stars at z > 10 and is quenching at z ∼ 8, resulting in a high star formation efficiency at high redshift (ϵ∼ 0.2 at z ∼ 5 and ϵ≳ 1 at z ≳ 8). As the most massive galaxy in its protocluster environment, UMG-28740 is a unique example of the impossibly early galaxy problem.