= Photoevaporation of a dark-matter minihalo
{title2=$t_{\rm evap}\sim R_h/c_{s,\rm ion}$}
<Photoionization> can heat a shallow halo's gas well above its <virial temperature>. If the heated <sound speed> exceeds the escape-speed scale, the previous <hydrostatic equilibrium> is lost and a pressure-driven outflow removes gas. A crossing time gives a useful estimate, but photon supply, the gas profile, shielding and recombinations can delay the process. This particularly affects haloes whose <virial temperatures> are below the usual atomic cooling threshold and reduces their subsequent <star formation>.
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