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 ± 0.07 at zₛpec = 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 Rₚroj ~ 1 cMpc. The Lyα profile is highly asymmetric (Af = 3.56), suggesting the presence of neutral gas within the interstellar medium, circumgalactic medium, or via AGN-driven outflows. 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 stellar mass of UMG-28740 to be log(M_/M₎ ~ 11 with a small standard deviation between measurements (σ = 0.07). 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.