The monopole and quadrupole moments of the Epoch of Reionization (EoR) 21-cm bispectrum

Abstract

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

Accepted for publication in MNRAS

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• The Effect of Large Optical Depths on the Non-Gaussian 21-cm signal from Cosmic Dawn