Anisotropic Magnification Distortion of the 3D Galaxy Correlation: II. Fourier and Redshift Space

TL;DR

This paper explores how gravitational lensing magnification biases distort the 3D galaxy correlation and power spectrum, especially in Fourier space, and discusses how to distinguish these effects from other anisotropies.

Contribution

It extends the analysis of magnification bias effects to the power spectrum and redshift space, highlighting their distinct shapes and implications for precision cosmology.

Findings

Magnification bias affects the galaxy power spectrum, especially in transverse Fourier modes.

Lensing anisotropy has a distinct shape from redshift space distortions, enabling potential separation.

In Fourier space, linearity is preserved at small wavenumbers, reducing magnification distortion severity.

Abstract

In paper I of this series we discuss how magnification bias distorts the 3D correlation function by enhancing the observed correlation in the line-of-sight (LOS) orientation, especially on large scales. This lensing anisotropy is distinctive, making it possible to separately measure the galaxy-galaxy, galaxy-magnification {\it and} magnification-magnification correlations. Here we extend the discussion to the power spectrum and also to redshift space. In real space, pairs oriented close to the LOS direction are not protected against nonlinearity even if the pair separation is large; this is because nonlinear fluctuations can enter through gravitational lensing at a small transverse separation (or i.e. impact parameter). The situation in Fourier space is different: by focusing on a small wavenumber $k$, as is usually done, linearity is guaranteed because both the LOS and transverse wavenumbers must be small. This is why magnification distortion of the galaxy correlation appears less severe in Fourier space. Nonetheless, the effect is non-negligible, especially for the transverse Fourier modes, and should be taken into account in interpreting precision measurements of the galaxy power spectrum, for instance those that focus on the baryon oscillations. The lensing induced anisotropy of the power spectrum has a shape that is distinct from the more well known redshift space anisotropies due to peculiar motions and the Alcock-Paczynski effect. The lensing anisotropy is highly localized in Fourier space while redshift space distortions are more spread out. This means that one could separate the magnification bias component in real observations, implying that potentially it is possible to perform a gravitational lensing measurement without measuring galaxy shapes.