  I use simulations of cosmological reionization to quantify the effect of
photoionization on the gas fraction in low mass objects, in particular the
characteristic mass scale below which the gas fraction is reduced compared to
the universal value. I show that this characteristic scale can be up to an
order of magnitude lower than the linear theory Jeans mass, and that even if
one defines the Jeans mass at a higher overdensity, it does not track the
evolution of this characteristic suppression mass. Instead, the filtering mass,
which corresponds directly to the scale over which baryonic perturbations are
smoothed in linear perturbation theory, provides a remarkably good fit to the
characteristic mass scale. Thus, it appears that the effect of reionization on
structure formation in both the linear and nonlinear regimes is described by a
single characteristic scale, the filtering scale of baryonic perturbations. In
contrast to the Jeans mass, the filtering mass depends on the full thermal
history of the gas instead of the instantaneous value of the sound speed, so it
accounts for the finite time required for pressure to influence the gas
distribution in the expanding universe. In addition to the characteristic
suppression mass, I study the full shape of the probability distribution to
find an object with a given gas mass among all the objects with the same total
mass, and I show that the numerical results can be described by a simple
fitting formula that again depends only on the filtering mass. This simple
description of the probability distribution may be useful for semi-analytical
modeling of structure formation in the early universe.
