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

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 (~9000 deg<SUP>2</SUP>) sky overlap. We explore this potential by combining the DESI 1% survey and DECaLS DR8. With ~106 deg<SUP>2</SUP> 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 $\theta &gt;8^{^{\prime }}(&lt;8^{^{\prime }})$. This brings stronger requirements for future systematics control. To fully realize such potential, we need to control the residual multiplicative shear bias |m| &lt; 0.006 and the bias in the mean redshift |Δz| &lt; 0.008, requiring the introduced bias in the measurement is &lt;0.31σ. 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 σ<SUB>R</SUB> ~ 0.04) and cosmic magnification.