2021ApJ...921...58S

We present the results of a deep K_s-band (2.1 µm) imaging survey of the Spitzer/HETDEX Exploratory Large-Area (SHELA) field using the NEWFIRM near-infrared camera on the KPNO Mayall 4 m telescope. This NEWFIRM HETDEX Survey reaches a 5σ depth of 22.4 AB mag (2''-diameter apertures corrected to total), is ∼50% and 90% complete at K ∼ 22.65 and K ∼ 22.15, respectively, and covers 22 deg² of the 24 deg² SHELA Spitzer/IRAC footprint (within "Stripe 82" of the Sloan Digital Sky Survey). We present a K_s-band-selected catalog that includes deep ugriz imaging from the Dark Energy Camera and 3.6 and 4.5 µm imaging from Spitzer/IRAC, with forced photometry of 1.7 million sources across 17.5 deg². The large area and moderate depth of this catalog enable the study of the most massive galaxies at high redshift and minimize uncertainties associated with counting statistics and cosmic variance. As a demonstration, we derive stellar masses (M_*) and star formation rates (SFRs) for candidate galaxies at 3 <= z <= 5 and select a conservative sample of nine candidate massive (M_* > 10¹¹ M_☉) quiescent galaxies, which have measured SFRs significantly below the main sequence at this redshift. Five are ultramassive with M_* > 10¹², though uncertainties in IRAC blending, gravitational lensing, or active galactic nucleus emission could result in true masses that are lower. Simulations predict that these galaxies should be extremely rare; thus, we conclude by discussing what physical processes in models could be altered to allow the formation of such massive quiescent galaxies at such early times.