Photo-evaporation by thermal winds in dwarf galaxies

We revisit the evaporation process of gas from dwarf galaxies after it has been photo-ionized by the UV flux from the first stars and AGNs and heated to T ≃ 10 or 2 × 10K respectively. Earlier estimates, based on the balance between pressure and gravity, indicated that dark haloes of virial velocity lower than Vevap ≃ 11−13 kms −1 have lost most of their gas in a dynamical time. We follow the continuous evaporation by a thermal wind during the period when the ionizing flux was effective. We find that the critical virial velocity for significant evaporation is significantly higher. For example, if the ionization starts at zion = 10 and is maintained until z = 2, a mass loss of one e-fold occurs in haloes of Vevap ≃ 25 (or 35 kms ) for T ≃ 10K (or 2×10K). Haloes of Vevap ≃ 21 kms −1 (or 29 kms) lose one e-fold within the first Hubble time at z = 10. Any dwarf galaxies with virial velocities smaller than Vevap must have formed their stars from a small fraction of their gas before zion, and then lost the rest of the gas by photo-evaporation. This may explain the gas-poor, low surface brightness dwarf spheroidal galaxies. By z < 1, most of the IGM gas was evaporated at least once form dwarf galaxies, thus providing a lower bound to its metallicity.