Forecasts for Dark Matter-Radiation Interactions with the Rubin Observatory's LSST

Interactions between dark matter (DM) and photons in the early Universe suppress structure formation on small scales. In particular, elastic scattering transfers heat and momentum from photons to DM, ultimately reducing the abundance of low-mass DM halos and the dwarf galaxies they host. This small-scale suppression ensures that populations like the Milky Way (MW) satellites are ideal laboratories for constraining such cosmologies. In this work, we consider temperature-independent interactions parameterized by DM mass (mχ) and DM–photon interaction cross section (σχ–). We forecast bounds on this DM–photon cross section based on the expected sensitivity of the upcoming Rubin Observatory’s Legacy Survey of Space and Time (LSST). Using the results of Nadler et al.—that LSST’s MW satellite observations are expected to yield a thermal relic Warm DM (WDM) mass cutoff of 12.9 keV—we map this mass scale onto photon scattering parameter space. By requiring that the linear matter power spectrum be less suppressed than in the case of this thermal-relic WDM cutoff, we predict that LSST will impose the following 95% upper limit at m=1 MeV: -<1.2510-39cm2. Our bounds on photon-DM cross section depend linearly on DM mass for m1 MeV. This dependence approaches m3 at lower masses due to the effects of DM sound speed. Across all masses, our forecasted upper limit on cross section is 1 order of magnitude more stringent than previous explorations of MW satellites using pre-LSST WDM cutoffs.