Constraints on dark matter-electron scattering from molecular cloud ionization

We demonstrate that ionization of H2 by dark matter in dense molecular clouds can provide strong constraints on the scattering strength of dark matter with electrons. Molecular clouds have high UV-optical attenuation, shielding them from ultraviolet and x-ray photons. Their chemical and thermal evolution are governed by low-energy cosmic rays. Dark matter with mass ≳4 MeV can ionize H2, contributing to the observed ionization rate. We require that the dark matter-induced ionization rate of H2 not exceed the observed cosmic-ray ionization rate ζH2, in diffuse molecular clouds as well as dense molecular clouds such as L1551 in the Taurus cloud complex. This allows us to place strong constraints on the dark matter-electron cross section σ¯e that complement existing astrophysical constraints and probe the strongly interacting parameter space where terrestrial and underground direct-detection experiments lose sensitivity. We show that constraints from molecular clouds combined with planned balloon and satellite-based experiments would strongly constrain the fractional abundance of dark matter that interacts strongly with electrons. We comment on future modeling and observational efforts that may improve our bounds.