Joint constraints on thermal relic dark matter from strong gravitational lensing, the Ly α forest, and Milky Way satellites

We derive joint constraints on the warm dark matter (WDM) half-mode scale by combining the analyses of a selection of astrophysical probes: strong gravitational lensing with extended sources, the Ly α forest, and the number of luminous satellites in the Milky Way. We derive an upper limit of λ_hm = 0.089 Mpc h^-1 at the 95 per cent confidence level, which we show to be stable for a broad range of prior choices. Assuming a Planck cosmology and that WDM particles are thermal relics, this corresponds to an upper limit on the half-mode mass of M_hm < 3 × 10⁷ M_⊙ h^-1, and a lower limit on the particle mass of m_th > 6.048 keV, both at the 95 per cent confidence level. We find that models with λ_hm > 0.223 Mpc h^-1 (corresponding to m_th > 2.552 keV and M_hm < 4.8 × 10⁸ M_⊙ h^-1) are ruled out with respect to the maximum likelihood model by a factor ≤1/20. For lepton asymmetries L₆ > 10, we rule out the 7.1 keV sterile neutrino dark matter model, which presents a possible explanation to the unidentified 3.55 keV line in the Milky Way and clusters of galaxies. The inferred 95 percentiles suggest that we further rule out the ETHOS-4 model of self-interacting DM. Our results highlight the importance of extending the current constraints to lower half-mode scales. We address important sources of systematic errors and provide prospects for how the constraints of these probes can be improved upon in the future.