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

Ke Ma, Haibin Zhang, Zheng Cai, Yongming Liang, Nobunari Kashikawa, Mingyu Li, Yunjing Wu, Qiong Li, Xiaohui Fan, Sean D. Johnson, Masami Ouchi

Research output: Contribution to journalArticlepeer-review


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 σg of galaxy numbers in observational astronomy, which significantly affects the luminosity function (LF) measurement of Lyα emitters (LAEs). For most previous Lyα LF studies, σg 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), L Ly α * = 5.95 − 0.96 + 1.22 × 10 42 erg s−1, and ϕ Ly α * = 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 σg by sampling LAEs within multiple pointings on the field image. We investigate the relation between σg and survey volume V, and fit a simple power-law σ g = k × V 10 5 Mpc 3 β . 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.

Original languageEnglish (US)
Article number102
JournalAstrophysical Journal
Issue number1
StatePublished - Jan 1 2024

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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