First direct astrophysical constraints on dark matter interactions at very low velocities

We derive the first direct constraints on millicharged DM cross sections at very low velocity, v ~ 7 km/s, by requiring that the heating/cooling due to DM interacting with gas in the Leo T dwarf galaxy not exceed the radiative cooling rate. We also use equilibrium heating/cooling constraints for durable Galactic molecular gas clouds to improve the constraints on sub-GeV DM cross sections for arbitrary velocity dependences. We correct erroneous results in the literature that were based on unsuitable clouds and analyses. Our results exclude the millicharge explanation for the reported EDGES 21 cm anomaly even when only 1% of DM is charged.