Angular spectra of the intrinsic galaxy ellipticity field, their observability and their impact on lensing in tomographic surveys

Abstract

Subject of this paper are intrinsic ellipticity correlations between galaxies, their statistical properties, their observability with future surveys and their interference with weak gravitational lensing measurements. Using an angular momentum-based, quadratic intrinsic alignment model we derive correlation functions of the ellipticity components and project them to yield the four non-zero angular ellipticity spectra $C^\epsilon_E(\ell)$, $C^\epsilon_B(\ell)$, $C^\epsilon_C(\ell)$ and $C^\epsilon_S(\ell)$ in their generalisation to tomographic surveys. For a Euclid-like survey, these spectra would have amplitudes smaller than the weak lensing effect on nonlinear structures, but would constitute an important systematic. Computing estimation biases for cosmological parameters derived from an alignment-contaminated survey suggests biases of $+5\sigma_w$ for the dark energy equation of state parameter $w$, $-20\sigma_{\Omega_m}$ for the matter density $\Omega_m$ and $-12\sigma_{\sigma_8}$ for the spectrum normalisation $\sigma_8$. Intrinsic alignments yield a signal which is easily observable with a survey similar to Euclid: While not independent, significances for estimates of each of the four spectra reach values of tens of $\sigma$ if weak lensing and shape noise are considered as noise sources, which suggests relative uncertainties on alignment parameters at the percent level.