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

Abstract

ABSTRACT We study the monopole ($\bar{B}^0_0$) and quadrupole ($\bar{B}^0_2$) moments of the 21-cm bispectrum (BS) from EoR simulations and present results for squeezed and stretched triangles. Both $\bar{B}^0_0$ and $\bar{B}^0_2$ are positive at the early stage of EoR where the mean neutral hydrogen (${\rm H\, {\small I}}$) density fraction $\bar{x}_{\rm {\rm H\, {\small I}}}\approx 0.99$. The subsequent evolution of $\bar{B}^0_0$ and $\bar{B}^0_2$ at large and intermediate scales (k = 0.29 and $0.56 \, {\rm Mpc}^{-1}$, respectively) is punctuated by two sign changes which mark transitions in the ${\rm H\, {\small I}}$ distribution. The first sign flip where $\bar{B}^0_0$ becomes negative occurs in the intermediate stages of EoR $(\bar{x}_{\rm {\rm H\, {\small I}}}\gt 0.5)$, at large scale first followed by the intermediate scale. This marks the emergence of distinct ionized bubbles in the neutral background. $\bar{B}^0_2$ is relatively less affected by this transition, and it mostly remains positive even when $\bar{B}^0_0$ becomes negative. The second sign flip, which affects both $\bar{B}^0_0$ and $\bar{B}^0_2$, occurs at the late stage of EoR $(\bar{x}_{\rm {\rm H\, {\small I}}}\lt 0.5)$. This marks a transition in the topology of the ${\rm H\, {\small I}}$ distribution, after which we have distinct ${\rm H\, {\small I}}$ islands in an ionized background. This causes $\bar{B}^0_0$ to become positive. The negative $\bar{B}^0_2$ is a definite indication that the ${\rm H\, {\small I}}$ islands survive only in underdense regions.