A Novel Test for MOND: Gravitational Lensing by Disk Galaxies

Abstract Disk galaxies represent a promising laboratory for the study of gravitational physics, including alternatives to dark matter (DM), owing to the possibility of coupling rotation curves’ dynamical data with strong gravitational lensing (SGL) observations. In particular, Euclid, DES, and LSST are predicted to observe hundreds of thousands of gravitational lenses. Here, we investigate disk galaxy SGL in the modified Newtonian dynamics (MOND) framework. We employ the concept of equivalent Newtonian systems within the quasi-linear MOND formulation to make use of the standard lensing formalism. We derive the phantom DM distribution predicted for realistic disk galaxy models and study the impact of morphological and mass parameters on the expected lensing. We find purely MONDian effects dominate the lensing—due to the presence of a phantom DM disk—and generate nontrivial correlations between the lens parameters and the lensing cross section. Moreover, we show that MOND predicts a substantial increase in the number count of disk galaxy lenses compared to the DM-driven predictions, making it distinguishable from the latter in upcoming surveys regardless of the chosen interpolating function. Finally, we argue that disk galaxy gravitational lensing, coupled to additional astronomical observations, can be used to constrain the interpolating function of MOND.