  Motivated by the recent direct detection of cosmological gas infall, we
develop an analytical model for calculating the mean density profile around an
initial overdensity that later forms a dark matter halo. We account for the
problem of peaks within peaks; when considering a halo of a given mass we
ensure that this halo is not a part of a larger virialized halo. For halos that
represent high-sigma fluctuations we recover the usual result that such halos
preferentially lie within highly overdense regions; in the limit of very
low-sigma fluctuations, on the other hand, we show that halos tend to lie
within voids. Combined with spherical collapse, our results yield the typical
density and velocity profiles of the gas and dark matter that surround
virialized halos.
