Dwarf Galaxy Halo Masses from Spectroscopic and Photometric Lensing in DESI and DES

We present the most precise and lowest-mass weak lensing measurements of dwarf galaxies to date, enabled by spectroscopic lenses from the Dark Energy Spectroscopic Instrument (DESI) and photometric lenses from the Dark Energy Survey (DES) calibrated with DESI redshifts. Using DESI spectroscopy from the first data release, we construct clean samples of galaxies with median stellar masses log 10 ( M * / M ⊙ ) =8.310.1 and measure their weak lensing signals with sources from DES, KiDS, and SDSS, achieving detections with signal-to-noise ratios (S/N) up to 12 for dwarf galaxies ( log 10 ( M * / M ⊙ ) < 9.25)—opening up a new regime for lensing measurements of low-mass systems. Leveraging DES photometry calibrated with DESI, we extend to a photometric dwarf sample of over 700,000 galaxies, enabling robust lensing detections of dwarf galaxies with combined S/N = 30 and a significant measurement down to log 10 ( M * / M ⊙ ) =8.0. The photometric sample is characterized using a self-organizing map calibrated with DESI spectroscopy, a methodology designed to scale to future surveys like Vera C. Rubin Observatory’s LSST where spectroscopic coverage will be relatively limited. We show that the one-halo regime (scales ≲0.15 h −1 Mpc) is insensitive to various systematic and sample selection effects, providing robust halo mass estimates, while the signal in the two-halo regime depends on galaxy color and environment. These results demonstrate that DESI already enables precise dwarf lensing measurements, and that calibrated photometric samples extend this capability. Together, they pave the way for novel constraints on dwarf galaxy formation and dark matter physics with upcoming surveys like LSST.