Galaxy Statistics in Pencil-beam Surveys at High Redshifts

Surveys of faint galaxies at high redshifts often result in a “pencil-beam” geometry that is much longer along the line-of-sight than across the sky. We explore the effects of this geometry on the abundance and clustering of Lyman-break galaxies (LBGs) and Lyman-α emitters (LAEs) based on cosmological N-body simulations which adequately describe the nonlinear growth of structure on small scales. We find that the probability distribution of the LBG abundance is skewed toward low values since the narrow transverse dimension of the survey is more likely to probe underdense regions. Over a range that spans 1–2 orders of magnitude in galaxy luminosities, the variance in the number of objects differs from the commonly used analytic prediction and is not dominated by Poisson noise. Additionally, nonlinear bias on small scales results in a one-dimensional power spectrum of LAEs using a James Webb Space Telescope field-of-view that is relatively flat, markedly different from the expectation of linear perturbation theory. We discuss how these results may affect attempts to measure the UV background at high redshifts, estimate the relationship between halo mass and galaxy luminosity, and probe reionization by measuring the power-modulating effect of ionized regions.