DESI and DECaLS (D&D): galaxy-galaxy lensing measurements with 1% survey and its forecast

TL;DR

This paper demonstrates the potential of galaxy-galaxy lensing measurements using DECaLS and DESI 1% survey data, highlighting significant detections and forecasting improved constraints with full samples, while emphasizing the need for systematic control.

Contribution

It introduces a method combining DECaLS and DESI data for galaxy-galaxy lensing, providing forecasts for full survey potential and systematic requirements.

Findings

Significant detection of galaxy-galaxy lensing with current data.

Forecasted error reduction to 1.3% at small scales with full samples.

Identified systematic control thresholds for shear and redshift biases.

Abstract

The shear measurement from DECaLS (Dark Energy Camera Legacy Survey) provides an excellent opportunity for galaxy-galaxy lensing study with DESI (Dark Energy Spectroscopic Instrument) galaxies, given the large ($\sim 9000$ deg$^2$) sky overlap. We explore this potential by combining the DESI 1\% survey and DECaLS DR8. With $\sim 106$ deg$^2$ sky overlap, we achieve significant detection of galaxy-galaxy lensing for BGS and LRG as lenses. Scaled to the full BGS sample, we expect the statistical errors to improve from $18(12)\%$ to a promising level of $2(1.3)\%$ at $\theta>8^{'}(<8^{'})$. This brings stronger requirements for future systematics control. To fully realize such potential, we need to control the residual multiplicative shear bias $|m|<0.01$ and the bias in the mean redshift $|\Delta z|<0.015$. We also expect significant detection of galaxy-galaxy lensing with DESI LRG/ELG full samples as lenses, and cosmic magnification of ELG through cross-correlation with low-redshift DECaLS shear. {If such systematical error control can be achieved,} we find the advantages of DECaLS, comparing with KiDS (Kilo Degree Survey) and HSC (Hyper-Suprime Cam), are at low redshift, large-scale, and in measuring the shear-ratio (to $\sigma_R\sim 0.04$) and cosmic magnification.