Quantifying the redshift space distortion of the bispectrum III : Detection prospects of the multipole moments

TL;DR

This paper develops a formalism to assess the detectability of bispectrum multipoles in galaxy surveys, showing Euclid can measure multipoles up to  with significant signal-to-noise, especially for low  multipoles.

Contribution

It introduces a method to compute statistical fluctuations and SNR for bispectrum multipoles, enabling better predictions of measurement prospects in galaxy surveys.

Findings

Euclid can potentially measure bispectrum multipoles up to .

SNR is highest for  multipoles at linear scales.

Measurement errors are weakly correlated across most triangle shapes.

Abstract

The redshift space anisotropy of the bispectrum is generally quantified using multipole moments. The possibility of measuring these multipoles in any survey depends on the level of statistical fluctuations. We present a formalism to compute the statistical fluctuations in the measurement of bispectrum multipoles for galaxy surveys. We consider specifications of a {\it Euclid} like galaxy survey and present two quantities: the signal-to-noise ratio (SNR) which quantifies the detectability of a multipole, and the rank correlation which quantifies the correlation in measurement errors between any two multipoles. Based on SNR values, we find that {\it Euclid} can potentially measure the bispectrum multipoles up to $\ell=4$ across various triangle shapes, formed by the three {\bf k} vectors in Fourier space. In general, SNR is maximum for the linear triangles. SNR values also depend on the scales and redshifts of observation. While, $\ell \leq 2$ multipoles can be measured with ${\rm SNR}>5$ even at linear/quasi-linear ($k \lesssim 0.1 \,{\rm Mpc}^{-1}$) scales, for $\ell>2$ multipoles, we require to go to small scales or need to increase bin sizes. For most multipole pairs, the errors are only weakly correlated across much of the triangle shapes barring a few in the vicinity of squeezed and stretched triangles. This makes it possible to combine the measurements of different multipoles to increase the effective SNR.