Steeling Weak Lensing Source Galaxy Samples against Systematics using Wide Field Spectroscopy

TL;DR

This paper demonstrates that weak lensing analyses can be robust against systematics by using less dense source galaxy samples calibrated with wide field spectroscopy, enhancing cosmological constraints.

Contribution

It introduces a method to mitigate systematics in weak lensing by combining flexible models and spectroscopic calibration, allowing sparser galaxy samples without losing constraining power.

Findings

Weak lensing information saturates on quasi-linear scales under current baryonic physics uncertainties.

Sparser source galaxy samples with densities around 5 per arcmin^2 are sufficient if redshift distributions are well calibrated.

Spectroscopic instruments like DESI can calibrate redshift and intrinsic alignment contamination, reducing dominant systematics.

Abstract

We investigate the cosmological constraining power of combined weak galaxy lensing and galaxy clustering probes, i.e. $3\times2$-point analyses, assuming flexible models for redshift uncertainty, and Lagrangian perturbation theory and hybrid effective field theory models for galaxy intrinsic alignments, galaxy bias and baryonic physics. In this context, we provide a detailed accounting of the limiting systematics on $3\times2$-point analyses. Our main finding is that in the presence of current levels of uncertainty on baryonic physics, the information content of weak lensing analyses saturates on quasi-linear scales, allowing the use of source galaxy samples that are significantly less dense, e.g. with number densities of $5\rm \, arcmin^{-2}$, without sacrificing constraining power, provided that redshift distributions can be calibrated at the $\sigma(\langle z\rangle)=0.005$ level. We show that for sufficiently narrow lens and source redshift distributions, intrinsic alignment contributions can be largely self-calibrated, though sufficient flexibility must be given to the redshift and scale dependence of this signal. The near optimality of such relatively sparse source galaxy samples opens the possibility to directly calibrate the redshift distributions and intrinsic alignment contamination of such a sample using a spectroscopic instrument like DESI, thus mitigating the dominant systematics in weak lensing analyses.