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 (M_/M_) = 10. 98 0. 07 at zₛpec = 4. 8947 in the Extended Groth Strip (EGS), based on deep observations of Ly 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 (M_/M_) > 10. 5 within Rₚroj 1 cMpc. The Ly 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 (M_/M_) 11 with a small standard deviation between measurements (= 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 (0. 2 at z 5 and 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.