Euclid preparation: XIX. Impact of magnification on photometric galaxy clustering

TL;DR

This paper demonstrates that lensing magnification significantly influences galaxy clustering measurements and cosmological parameter estimates in Euclid, emphasizing the necessity of including this effect for accurate analysis.

Contribution

It provides a detailed assessment of lensing magnification's impact on Euclid's galaxy clustering and cross-correlation analyses, highlighting the importance of incorporating this effect in future cosmological studies.

Findings

Magnification reduces errors on key cosmological parameters by 20-35%.

Neglecting magnification causes parameter shifts up to 1.6σ.

Ignoring magnification biases the combined analysis by up to 6σ.

Abstract

We investigate the importance of lensing magnification for estimates of galaxy clustering and its cross-correlation with shear for the photometric sample of Euclid. Using updated specifications, we study the impact of lensing magnification on the constraints and the shift in the estimation of the best fitting cosmological parameters that we expect if this effect is neglected. We follow the prescriptions of the official Euclid Fisher matrix forecast for the photometric galaxy clustering analysis and the combination of photometric clustering and cosmic shear. The slope of the luminosity function (local count slope), which regulates the amplitude of the lensing magnification, and the galaxy bias have been estimated from the Euclid Flagship simulation.We find that magnification significantly affects both the best-fit estimation of cosmological parameters and the constraints in the galaxy clustering analysis of the photometric sample. In particular, including magnification in the analysis reduces the 1$\sigma$ errors on $\Omega_{\text{m},0}, w_{0}, w_a$ at the level of 20-35%, depending on how well we will be able to independently measure the local count slope. In addition, we find that neglecting magnification in the clustering analysis leads to shifts of up to 1.6$\sigma$ in the best-fit parameters. In the joint analysis of galaxy clustering, cosmic shear, and galaxy-galaxy lensing, magnification does not improve precision, but it leads to an up to 6$\sigma$ bias if neglected. Therefore, for all models considered in this work, magnification has to be included in the analysis of galaxy clustering and its cross-correlation with the shear signal ($3\times2$pt analysis) for an accurate parameter estimation.