Exploring Cosmic Dawn with PANORAMIC. II. Cosmic Variance and Galaxy Clustering at z ∼ 10

Abstract Observational campaigns with JWST have revealed a higher-than-expected abundance of UV-bright galaxies at z ≳ 10, with various proposed theoretical explanations. A powerful complementary constraint to break degeneracies between different models is galaxy clustering. In this paper, we combine PANORAMIC pure parallel and legacy imaging along 34 independent sight lines to measure the cosmic variance ( σ CV ) in the number counts of Lyman break galaxies at z ∼ 10, which is directly related to their clustering strength. We find σ CV = 0.9 6 − 0.18 + 0.20 , 1.4 6 − 0.44 + 0.54 , and 1.7 1 − 0.59 + 0.72 per NIRCam pointing ( ∼ 9.7 arcmin 2 , ≲1.5 pMpc at z ∼ 10) for galaxies with M UV < −19.5, −20, and −20.5. Comparing to galaxies in the fiducial UniverseMachine , we find that σ CV is consistent with our measurements, but that the number densities are a factor ≳5 lower. We implement simple models in the UniverseMachine that represent different physical mechanisms to enhance the number density of UV-bright galaxies. All models decrease σ CV by placing galaxies at fixed M UV in lower-mass halos, but to varying degrees. Combined constraints on σ CV and the UV luminosity function thus tentatively disfavor models that globally increase the star formation efficiency (SFE) or the scatter in the M UV – M halo relation, while models that decrease the mass-to-light ratio, or assume a power-law scaling of the SFE with M halo agree better with the data. We show that with sufficient additional independent sight lines, robust discrimination between models is possible, paving the way for powerful constraints on the physics of early galaxy evolution through NIRCam pure parallel imaging.