DESI and DECaLS (D&D): galaxy–galaxy lensing measurements with 1 per cent survey and its forecast

ABSTRACT The shear measurement from the Dark Energy Camera Legacy Survey (DECaLS) provides an excellent opportunity for galaxy–galaxy lensing study with the Dark Energy Spectroscopic Instrument (DESI) galaxies, given the large (∼9000 deg2) sky overlap. We explore this potential by combining the DESI 1 per cent survey and DECaLS Data Release 8 (DR8). With ∼106 deg2 sky overlap, we achieve significant detection of galaxy–galaxy lensing for Bright Galaxy Survey (BGS) and luminous red galaxy (LRG) as lenses. Scaled to the full BGS sample, we expect the statistical errors to improve from $18(12)\,{\rm per \ cent}$ to a promising level of $2(1.3)\,{\rm per \ cent}$ at $\theta \gt 8\,\mathrm{ arcmin} \, (\lt 8\,\mathrm{ arcmin})$. This brings stronger requirements for future systematics control. To fully realize such potential, we need to control the residual multiplicative shear bias |m| < 0.006 and the bias in the mean redshift |Δz| < 0.008, requiring the introduced bias in the measurement is