Free-Streaming Length of Dark Matter from JWST Observations of 28 Strong Gravitational Lenses

The formation of gravitationally bound overdensities of dark matter (DM), or halos, is a generic prediction of theories with particle DM. We present a measurement of halo properties in 28 quadruple image strong lens systems recently observed by JWST, and use these observations to constrain the free-streaming length, λ_{FS}, of DM, a quantity that depends on the DM particle mass and formation mechanism. We improve on previous lensing analyses by simultaneously reconstructing extended lensed arcs with image positions and relative magnifications, enhancing sensitivity to perturbations by halos. Our analysis rules out deviations from the predictions of cold dark matter (CDM) on scales above 10^{7.2}M_{⊙} and 10^{7.4}M_{⊙} for subhalo abundance predicted by cosmological N-body simulations and semianalytic models, respectively. These bounds correspond to upper limits λ_{FS}<6.0  kpc and λ_{FS}<7.0  kpc, and lower limits on the mass of a spin-1/2 thermal relic DM particle m_{therm}>7.4  keV and m_{therm}>6.5  keV. Conversely, assuming a negligible free-streaming length, as predicted by CDM, we measure a projected mass in subhalos around elliptical galaxies 1.7_{-1.2}^{+2.6}×10^{7}  M_{⊙} kpc^{-2} at 95% confidence. These inferences confirm key predictions of the CDM paradigm.