The monopole and quadrupole moments of the epoch of reionization (EoR) 21-cm bispectrum

Abstract

We study the monopole (B̄00) and quadrupole (B̄20) moments of the 21-cm bispectrum (BS) from EoR simulations and present results for squeezed and stretched triangles. Both B̄00 and B̄20 are positive at the early stage of EoR where the mean neutral hydrogen (H I) density fraction x̄H I ≈ 0.99. The subsequent evolution of B̄00 and B̄20 at large and intermediate scales (k = 0.29 and 0.56 Mpc−1, respectively) is punctuated by two sign changes which mark transitions in the H I distribution. The first sign flip where B̄00 becomes negative occurs in the intermediate stages of EoR (x̄H I &gt

0.5), at large scale first followed by the intermediate scale. This marks the emergence of distinct ionized bubbles in the neutral background. B̄20 is relatively less affected by this transition, and it mostly remains positive even when B̄00 becomes negative. The second sign flip, which affects both B̄00 and B̄20, occurs at the late stage of EoR (x̄H I &lt

0.5). This marks a transition in the topology of the H I distribution, after which we have distinct H I islands in an ionized background. This causes B̄00 to become positive. The negative B̄20 is a definite indication that the H I islands survive only in underdense regions. © 2023 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society.