# Weak lensing power spectrum reconstruction by counting galaxies.-- I: the ABS method

## TL;DR

This paper introduces the ABS method for reconstructing the cosmic lensing power spectrum by analyzing galaxy counts across flux bins, avoiding assumptions about galaxy clustering and cosmology.

## Contribution

The paper presents an analytical, assumption-free method to separate cosmic magnification from intrinsic galaxy clustering using cross power spectra, applicable to large surveys.

## Key findings

- Effective reconstruction at z~1 and l<5000 for SKA and LSST
- Method is unbiased with infinite galaxy samples
- Performance depends on survey specifics and shot noise

## Abstract

We propose an Analytical method of Blind Separation (ABS) of cosmic magnification from the intrinsic fluctuations of galaxy number density in the observed galaxy number density distribution. The ABS method utilizes the different dependences of the signal (cosmic magnification) and contamination (galaxy intrinsic clustering) on galaxy flux, to separate the two. It works directly on the measured cross galaxy angular power spectra between different flux bins. It determines/reconstructs the lensing power spectrum analytically, without assumptions of galaxy intrinsic clustering and cosmology. It is unbiased in the limit of infinite number of galaxies. In reality the lensing reconstruction accuracy depends on survey configurations, galaxy biases, and other complexities, due to finite number of galaxies and the resulting shot noise fluctuations in the cross galaxy power spectra. We estimate its performance (systematic and statistical errors) in various cases. We find that, stage IV dark energy surveys such as SKA and LSST are capable of reconstructing the lensing power spectrum at $z\simeq 1$ and $\ell\la 5000$ accurately. This lensing reconstruction only requires counting galaxies, and is therefore highly complementary to the cosmic shear measurement by the same surveys.