  We highlight a viable mechanism leading to the formation of dark energy
structures on sub-horizon cosmological scales, starting from linear
perturbations in scalar-tensor cosmologies. We show that the coupling of the
dark energy scalar field, or Quintessence, to the Ricci scalar induces a
"dragging" of its density perturbations through the general relativistic
gravitational potentials. We discuss, in particular, how this process forces
dark energy to behave as a pressureless component if the cosmic evolution is
dominated by non-relativistic matter. This property is also analyzed in terms
of the effective sound speed of the dark energy, which correspondingly
approaches the behavior of the dominant cosmological component, being
effectively vanishing after matter-radiation equality. To illustrate this
effect, we consider Extended Quintessence scenarios involving a quadratic
coupling between the field and the Ricci scalar. We show that Quintessence
density perturbations reach non-linearity at scales and redshifts relevant for
the structure formation process, respecting all the existing constraints on
scalar-tensor theories of Gravity. This study opens new perspectives on the
standard picture of structure formation in dark energy cosmologies, since the
Quintessence field itself, if non-minimally coupled to Gravity, may undergo
clustering processes, eventually forming density perturbations exiting from the
linear regime. A non-linear approach is then required to further investigate
the evolution of these structures, and in particular their role in the dark
haloes surrounding galaxies and clusters.
