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

Abstract

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$\hat{I}\pm$ 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. The Ly$\hat{I}\pm$ profile is highly asymmetric ($A_f=3.56$), 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 $\log (M_{\star }/M_{\odot })\sim 11$ with a small standard deviation between measurements ($\ddot{I}ƒ=0.07$). 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 $z\sim 8$, resulting in a high star formation efficiency at high redshift ($\hat{I}\mu \sim 0.2$ at $z\sim 5$ and $\hat{I}\mu \gtrsim 1$ 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.

11 pages, 6 figures, 2 tables, Accepted to OJA