Spectroscopic Confirmation of an Ultra-Massive Galaxy in a Protocluster at \(z \sim 4.9\)

We present spectroscopic confirmation of an ultra-massive galaxy (UMG) with \(\log(M_\star/M_\odot) = 10.98 \pm 0.07\) at \(z_\mathrm{spec} = 4.8947\) in the Extended Groth Strip (EGS), based on deep observations of Ly\(\alpha\) 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 \(\log(M_\star/M_\odot) > 10.5\) within \(R_\mathrm{proj} \sim 1\) 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 \(\log(M_\star/M_\odot) \sim 11\) with a small standard deviation between measurements (\(\sigma = 0.07\)). 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. \(\mathit{JWST}\)/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 \(z \sim 8\), resulting in a high star formation efficiency at high redshift (\(\epsilon \sim 0.2\) at \(z \sim 5\) and \(\epsilon \gtrsim 1\) at \(z \gtrsim 8\)). As the most massive galaxy in its protocluster environment, UMG-28740 is a unique example of the impossibly early galaxy problem.