MAMMOTH-Subaru. V. Effects of Cosmic Variance on Lyα Luminosity Functions at z = 2.2–2.3

Abstract

Cosmic variance introduces significant uncertainties into galaxy number density properties when surveying the high-redshift Universe with a small volume. Such uncertainties produce the field-to-field variance σ g of galaxy numbers in observational astronomy, which significantly affects the luminosity function (LF) measurement of Lyα emitters (LAEs). For most previous Lyα LF studies, σ g is often adopted from predictions by cosmological simulations, but barely confirmed by observations. Measuring cosmic variance requires a huge sample over a large volume, exceeding the capabilities of most astronomical instruments. In this study, we demonstrate an observational approach for measuring the cosmic variance contribution for z ≈ 2.2 Lyα LFs. The LAE candidates are observed using the narrow band and broad band of the Subaru/Hyper Suprime-Cam in eight independent fields, making the total survey area ≃11.62 deg2 and a comoving volume of ≃8.71 × 106 Mpc3. We report a best-fit Schechter function with parameters α = −1.75 (fixed), LLyα*=5.95−0.96+1.22×1042 erg s−1, and ϕLyα*=5.26−1.27+1.65 × 10−4 Mpc−3 for the overall Lyα LFs. After clipping out the regions that may bias the cosmic variance measurements, we calculate σ g by sampling LAEs within multiple pointings on the field image. We investigate the relation between σ g and survey volume V, and fit a simple power-law σg=k×V105Mpc3β . We find best-fit values of −1.399−0.156+0.160 for β and 1.249−0.193+0.213 for k. We compare our measurements with predictions from simulations and find that the cosmic variance of LAEs is likely larger than that of general star-forming galaxies.