A constraint on light primordial black holes from the interstellar medium temperature

Primordial black holes are a viable dark matter candidate. They decay via Hawking evaporation. Energetic particles from the Hawking radiation interact with interstellar gas, depositing their energy as heat and ionization. For a sufficiently high Hawking temperature, fast electrons produced by black holes deposit a substantial fraction of energy as heat through the Coulomb interaction. Using the dwarf galaxy Leo T, we place an upper bound on the fraction of primordial black hole dark matter. For $M \lt 5 \times 10^{-17}\, \mathrm{M}_\odot$, our bound is competitive with or stronger than other bounds.