MAMMOTHSubaru. V. Effects of Cosmic Variance on Lyα Luminosity Functions at z = 2.22.3
Abstract
Cosmic variance introduces significant uncertainties into galaxy number density properties when surveying the highredshift Universe with a small volume. Such uncertainties produce the fieldtofield 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 SuprimeCam in eight independent fields, making the total survey area ≃11.62 deg^{2} and a comoving volume of ≃8.71 × 10^{6} Mpc^{3}. We report a bestfit Schechter function with parameters α = 1.75 (fixed), ${L}_{\mathrm{Ly}\alpha }^{* }={5.95}_{0.96}^{+1.22}\times {10}^{42}$ erg s^{1}, and ${\phi }_{\mathrm{Ly}\alpha }^{* }={5.26}_{1.27}^{+1.65}$ × 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 powerlaw ${\sigma }_{g}=k\times {\left(\tfrac{V}{{10}^{5}\,{\mathrm{Mpc}}^{3}}\right)}^{\beta }$ . We find bestfit 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 starforming galaxies.
 Publication:

The Astrophysical Journal
 Pub Date:
 January 2024
 DOI:
 10.3847/15384357/ad04da
 arXiv:
 arXiv:2301.07360
 Bibcode:
 2024ApJ...961..102M
 Keywords:

 Lymanalpha galaxies;
 Cosmology;
 Luminosity function;
 978;
 343;
 942;
 Astrophysics  Astrophysics of Galaxies;
 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 14 pages, 7 figures