  Unified dark matter models are appealing in that they describe the dark
sector in terms of a single component. They however face problems when
attempting to account for structure formation: in the linear regime, density
fluctuations can become Jeans stable and oscillate rather than collapse, though
it is possible that this difficulty may be circumvented by invoking nonlinear
clustering. Here we examine the behaviour in the fully nonlinear regime, of
collapsed objects that should mimic standard dark matter haloes. It is shown
that the pressure gradient associated with the unified dark matter fluid should
be significant in the outer parts of galaxies and clusters, and its effects
obervable. In this case, no flat or falling rotation curve is possible for any
(barotropic) equation of state with associated sound speed decreasing with
density (a necessary condition if the fluid is to behave as pressureless matter
at high density). The associated density profile is therefore also incompatible
with that inferred in the outer part of clusters. For the prototypical case of
the generalised Chaplygin gas, it is shown that this limits the values of the
equation of state index $\alpha$ that are compatible with observations to
$\alpha \la 0.0001$ or $\alpha \ga 2$. This is in line from what is deduced
from linear analysis. More generally, from the expected properties of dark
matter haloes, constraints on the sound speed are derived. For the particular
case of the generalised Chaplygin gas, this further constrains the index to
$\alpha \la 10^{-9}$ or $\alpha \ga 6.7$. For a unified dark matter fluid to
mimic dark halo properties, therefore, it needs to have an equation of state
such that the pressure gradients are either minimal or which decrease fast
enough so as to be negligible at densities characteristic of the outer parts of
haloes.
