Abstract
We derive the first direct constraints on dark matter cross sections at very
low velocity, v $\approx$ 10 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 of the gas. This gives strong direct limits on millicharged DM
with mass $łesssim$ 1 GeV, which close important gaps in the exclusion plots
and add to constraints on the recent EDGES 21 cm absorption anomaly. We also
set new bounds on ultra-light dark photon DM and on DM-electron interactions.
Combining equilibrium heating/cooling constraints from Leo T and robust
Galactic gas clouds, we improve the constraints on DM-baryon cross-sections for
arbitrary velocity dependences.
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